({
    doInit : function(component, event, helper) {
        var action = component.get("c.fieldCheker");
        var recordId=component.get("v.recordId");
        action.setParams({
            "recId" : recordId
        });
        action.setCallback(this, function(response){
            var state = response.getState(); 
            if(state == 'SUCCESS') {                
                var returnValue= response.getReturnValue() 
                if(returnValue!=null){
                    if(returnValue.Promo__c==null || returnValue.Promo__c =='undefined'){
                        //alert("To convert this lead, Please specify whether Marketing Promo is applied or not.\n\nYou can update this information using the 'Promo' field under 'Marketing Information' section.\n\nThank you for your co-operation.")
                        var toastEvent = $A.get("e.force:showToast");
                        toastEvent.setParams({
                            title: "Error!",
                            message: "To convert this lead, Please specify whether Marketing Promo is applied or not.\n\nYou can update this information using the 'Promo' field under 'Marketing Information' section.\n\nThank you for your co-operation.",
                            type: "Error"});
                        //$A.get("e.force:closeQuickAction").fire()
                        //dismissActionPanel.fire();
                        //$A.get("e.force:refreshView").fire();
                        //toastEvent.fire(); 
                        //$A.get("e.force:refreshView").fire();
                        /*var svg = component.find("svg_content");
        var value = svg.getElement().innerText;
        value = value.replace("<![CDATA[", "").replace("]]>", "");
        svg.getElement().innerHTML = value;*/
                        component.set('v.myText','To convert this lead, Please specify whether Marketing \'Promo\' is applied or not.\nYou can update this information using the Promo field under \'Marketing Information\' section.\nThank you for your co-operation');
                    }
                    else if(returnValue.Flag_Type__c == 'Related'){
                       component.set('v.myText','You don\'t have sufficient access to convert a related lead.\nPlease swap this lead to master by clicking the swap to master button\nFor more information contact SMO team.');
                    }
                        else{
                            var navLink = component.find("navigation");
                            var pageRef = {
                                type: 'standard__objectPage',
                                attributes: {
                                    actionName: "convert",
                                    objectApiName: 'Lead',
                                    recordId : recordId  
                                },
                            };
                            navLink.navigate(pageRef, true);
                        }
                }
            }
        });
        if(recordId !=null && 'undefined' ){
            $A.enqueueAction(action);
        }
        
    }
})

    ({
    doInit : function(component, event, helper) {
        var action = component.get("c.fieldCheker");
        var recordId=component.get("v.recordId");
        action.setParams({
            "recId" : recordId
        });
        action.setCallback(this, function(response){
            var state = response.getState(); 
            if(state == 'SUCCESS') {                
                var returnValue= response.getReturnValue() 
                if(returnValue!=null){
                    if(returnValue.Promo__c==null || returnValue.Promo__c =='undefined'){
                        //alert("To convert this lead, Please specify whether Marketing Promo is applied or not.\n\nYou can update this information using the 'Promo' field under 'Marketing Information' section.\n\nThank you for your co-operation.")
                        var toastEvent = $A.get("e.force:showToast");
                        toastEvent.setParams({
                            title: "Error!",
                            message: "To convert this lead, Please specify whether Marketing Promo is applied or not.\n\nYou can update this information using the 'Promo' field under 'Marketing Information' section.\n\nThank you for your co-operation.",
                            type: "Error"});
                        //$A.get("e.force:closeQuickAction").fire()
                        //dismissActionPanel.fire();
                        //$A.get("e.force:refreshView").fire();
                        //toastEvent.fire(); 
                        //$A.get("e.force:refreshView").fire();
                        /*var svg = component.find("svg_content");
        var value = svg.getElement().innerText;
        value = value.replace("<![CDATA[", "").replace("]]>", "");
        svg.getElement().innerHTML = value;*/
                        component.set('v.myText','To convert this lead, Please specify whether Marketing \'Promo\' is applied or not.\nYou can update this information using the Promo field under \'Marketing Information\' section.\nThank you for your co-operation');
                    }
                    else if(returnValue.Flag_Type__c == 'Related'){
                       component.set('v.myText','You don\'t have sufficient access to convert a related lead.\nPlease swap this lead to master by clicking the swap to master button\nFor more information contact SMO team.');
                    }
                        else{
                            /*var navLink = component.find("navigation");
                            var pageRef = {
                                type: 'standard__objectPage',
                                attributes: {
                                    actionName: "convert",
                                    objectApiName: 'Lead',
                                    recordId : recordId  
                                },
                            };
                            navLink.navigate(pageRef, true);*/
                            var urlString = window.location.href;
                            var baseURL = 'https://testaz--current.lightning.force.com';
                            //use apex or JS libraries to get the base URL
                            // urlString.substring(0, urlString.indexOf("/lightning"));
                            var urlEvent = $A.get("e.force:navigateToURL");
                            urlEvent.setParams({
                            //ws is the relative url to the redirect screen once lead is converted. right now it will got to account page
                            "url": baseURL + '/lightning/cmp/runtime_sales_lead__convertDesktopConsole?leadConvert__leadId=' + recordId + 'ws=%2Flightning%2F%2FLead%2F'+recordId+'%2Fview'
                            });
                            urlEvent.fire();
                        }
                }
            }
        });
        if(recordId !=null && 'undefined' ){
            $A.enqueueAction(action);
        }
        
    }
})

Vf page:

<apex:page sidebar="false" showHeader="false" extensions="ConvertLeads" standardController="Lead"  action="{!updateconvert}">
<apex:pagemessages />

public with sharing class ConvertLeads {
    public ConvertLeads(ApexPages.StandardController stdCtrl) {
    }
    public Pagereference updateconvert()
    {
        String conId = ApexPages.currentPage().getParameters().get('id');
        Lead Related=[Select id,UniqueKey__c,Promo__c,Qualification_Score__c,Flag_Type__c from lead where id=:conId LIMIT 1];
        if(Related.Promo__c == null)
        {
            ApexPages.addMessage(new ApexPages.Message(ApexPages.Severity.Warning, 'To convert this lead, Please specify whether Marketing \'Promo\' is applied or not'+'\n'+'\n'+'You can update this information using the Promo field under \'Marketing Information\' section.'+'\n'+'\n'+'Thank you for your co-operation.'));
            return null;
        }
        
        else if(integer.valueof(Related.Qualification_Score__c.replace('%',''))<70 )
        {
            ApexPages.addMessage(new ApexPages.Message(ApexPages.Severity.Warning, 'Lead not qualified enough to convert'));
           return null;
        }
        else if(Related.Flag_Type__c == 'Related')
        {
            ApexPages.addMessage(new ApexPages.Message(ApexPages.Severity.Warning, 'You don\'t have sufficient access to convert a related lead.'+'\n'+'\n'+'Please swap this lead to master by clicking the swap to master button'+'\n'+'\n'+'For more information contact SMO team.'));
            return null;
        }
        return null;
    }
}
</apex:page>

My Component:

    <aura:component implements="force:appHostable,flexipage:availableForAllPageTypes,flexipage:availableForRecordHome,force:hasRecordId,force:lightningQuickAction" access="global"  controller="JavaScript_migration_controller">
    <aura:handler name="init" value="{!this}" action="{!c.doInit}"/>
    <aura:attribute name="myText" type="string" default=""/>
     <aura:attribute name="url" type="String" />
    <aura:attribute name="pageReference" type="Object" />
 <lightning:navigation aura:id="navigation"/>
    <div class="slds-notify slds-notify_alert slds-theme_alert-texture slds-theme_error" role="alert" aura:id="alerta">
  <span class="slds-assistive-text">error</span>
  <span class="slds-icon_container slds-icon-utility-error slds-m-right_x-small" title="Description of icon when needed">
  </span>
    
    <ui:outputText value="{!v.myText}"/>
</div>
</aura:component>

    ({
    doInit : function(component, event, helper) {
        var action = component.get("c.fieldCheker");
        var recordId=component.get("v.recordId");
        action.setParams({
            "recId" : recordId
        });
        action.setCallback(this, function(response){
            var state = response.getState(); 
            if(state == 'SUCCESS') {                
                var returnValue= response.getReturnValue() 
                if(returnValue!=null){
                    if(returnValue.Promo__c==null || returnValue.Promo__c =='undefined'){
                        //alert("To convert this lead, Please specify whether Marketing Promo is applied or not.\n\nYou can update this information using the 'Promo' field under 'Marketing Information' section.\n\nThank you for your co-operation.")
                        var toastEvent = $A.get("e.force:showToast");
                        toastEvent.setParams({
                            title: "Error!", 
                            message: "To convert this lead, Please specify whether Marketing Promo is applied or not.\n\nYou can update this information using the 'Promo' field under 'Marketing Information' section.\n\nThank you for your co-operation.",
                            type: "Error"});
                        //$A.get("e.force:closeQuickAction").fire()
                        //dismissActionPanel.fire();
                        //$A.get("e.force:refreshView").fire();
                        //toastEvent.fire(); 
                        //$A.get("e.force:refreshView").fire();
                        /*var svg = component.find("svg_content");
        var value = svg.getElement().innerText;
        value = value.replace("<![CDATA[", "").replace("]]>", "");
        svg.getElement().innerHTML = value;*/
                        component.set('v.myText','To convert this lead, Please specify whether Marketing \'Promo\' is applied or not.\nYou can update this information using the Promo field under \'Marketing Information\' section.\nThank you for your co-operation');
                    }
                    else if(returnValue.Flag_Type__c == 'Related'){
                       component.set('v.myText','You don\'t have sufficient access to convert a related lead.\nPlease swap this lead to master by clicking the swap to master button\nFor more information contact SMO team.');
                    }
                        else{
                            var navLink = component.find("navigation");
                            var pageRef = {
                                type: 'standard__objectPage',
                                attributes: {
                                    actionName: "convert",
                                    objectApiName: 'Lead',
                                    recordId : recordId  
                                },
                            };
                            navLink.navigate(pageRef, true);
                        }
                }
            }
        });
        if(recordId !=null && 'undefined' ){
            $A.enqueueAction(action);
        }
        
    }
})