VisualForce InputField values not being passed into Controller

Question

I am having trouble passing InputField data from a visualforce page into an apex controller. I feel like I have gone through 100 posts but nothing has helped me out. The goal here is to pass a new "Expiration" Date and "PO" attachment into the controller for it to add to existing records. When I bring up the VF page, attach the PO, add the date and click save, the controller yells at me saying the PO and date are null. I have attached my controller and VF Page code here.

Controller (the set Expiration Date part is commented out for debugging purposes):

public class ClosedWonMSAApex {

public Attachment file{get;set;}
public Opportunity opExpDate{get;set;}
Opportunity theOpportunity = new Opportunity();   

public ClosedWonMSAApex(ApexPages.StandardController stdController) {        
    this.theOpportunity = (Opportunity)stdController.getRecord();
} 


List<Opportunity> op = [ 
    SELECT Pricebook2Id, 
    LeadSource, 
    PrimaryContact__c , 
    Type,
    Loss_Reason__c, 
    MSA_Expiration_Date__c,
    CloseDate,
    Name, 
    StageName, 
    AccountId 
    FROM Opportunity 
    WHERE Id = :ApexPages.currentPage().getParameters().get('id')];



public PageReference save(){

    //Get values form VisualForce
    Opportunity opExpDate = new Opportunity();
    Attachment file  = new Attachment();

    //Create the new Op------------------------------------------------------------------------------------
    String newName;
    newName = '' + op[0].Name;
    newName = newName.substring(0,newName.length()-4);
    integer tempDate; 
    tempDate = op[0].CloseDate.year();
    tempDate++;
    newName = newName + tempDate;

    Opportunity newOP = new Opportunity(
        Pricebook2Id = op[0].Pricebook2Id, 
        LeadSource = op[0].LeadSource, 
        PrimaryContact__c = op[0].PrimaryContact__c,
        Type = op[0].Type,
        Loss_Reason__c  = op[0].Loss_Reason__c,
        CloseDate = op[0].CloseDate.addYears(1),
        Name = newName,
        StageName = 'AM - MSA Scheduled',
        AccountId = op[0].AccountId    
    );

    //Set the new Expiration date
    // if(op[0].MSA_Expiration_Date__c == null)
    // {
    //     newOP.MSA_Expiration_Date__c = opExpDate.MSA_Expiration_Date__c;
    // }

    // else
    // {
    //   newOP.MSA_Expiration_Date__c = op[0].MSA_Expiration_Date__c.addYears(1);
    // }       
    insert(newOP);

    //Attach the file-------------------------------------------------------------------------------------
    if(file != null)
    {
        Attachment attach = new Attachment(ParentId = op[0].Id, Name = file.name, Body = file.body);
        insert attach;
    }

    //Update the Old Opportunity--------------------------------------------------------------------------
    op[0].StageName = 'Closed Won';
    update(op[0]);

    return null;
}

}

VisualForce:

<apex:page standardController="Opportunity" extensions="ClosedWonMSAApex" showQuickActionVfHeader="false">  
<apex:form >
    <apex:pageBlock >
        <apex:pageBlockSection rendered="{!If(Opportunity.MSA_Expiration_Date__c == null,true,false) }" title="What is the new expiration date for this system(s)?">
            <apex:inputField value="{!opExpDate.MSA_Expiration_Date__c}" id="opExpDate"/>   
        </apex:pageBlockSection>
        <apex:pageBlockSection title="Attachment">
            Attach PO: <apex:inputFile value="{!file.body}" fileName="{!file.name}"></apex:inputFile>
        </apex:pageBlockSection>
    </apex:pageBlock>
    <apex:commandButton value="Save and Close" action="{!save}"/>        
</apex:form>  

Answer 1

You need to initialize both variable in your controller. As you have didn't initialize them so they are default null and throwing exception.

public ClosedWonMSAApex(ApexPages.StandardController stdController) { this.theOpportunity = (Opportunity)stdController.getRecord();
File = new attachment ();
Opexpdate = new opportunity ();
}