System.NullPointerException: Attempt to de-reference a null object

Question

I am getting this error when I am running my visualforce page

Below is my apex extension class: public with sharing class ExtensionAccount {

    public ExtensionAccount(ApexPages.StandardController controller) {

    }
    private final Account acct;  

    // The constructor passes in the standard controller defined
    // in the markup below
    public ExtensionAccount(ApexPages.StandardSetController controller) {
        this.acct = (Account)controller.getRecord(); 
    }    

    public ApexPages.StandardSetController TaskRecords {
        get {
            if(TaskRecords == null) {
                TaskRecords = new ApexPages.StandardSetController(
                    Database.getQueryLocator([SELECT AccountId FROM Task Where WhatId IN 
                        (Select id From Opportunity Where AccountId = :acct.Id)]));
            }
            return TaskRecords ;
        }
        private set;
    }
    public List<Task> getTaskList() {
         return (List<Task>) TaskRecords.getRecords();
    }  
}

Visualforce page

<apex:page standardController="Account" extensions="ExtensionAccount" sidebar="false"> 
<apex:pageBlock title="Opportunity"> 
<apex:pageBlockSection > 
<apex:dataList value="{!TaskList}" var="task" type="1"> {!task.accountid} </apex:dataList> 
</apex:pageBlockSection> 
</apex:pageBlock> 
</apex:form> 
</apex:page>

Answer 1

Since you are using single account id then add this.acct = (Account)controller.getRecord(); in your StandardController

public ExtensionAccount(ApexPages.StandardController controller) {
       this.acct = (Account)controller.getRecord(); //Add this line
    }
    private final Account acct;  

    // The constructor passes in the standard controller defined
    // in the markup below
    public ExtensionAccount(ApexPages.StandardSetController controller) {
        this.acct = (Account)controller.getRecord(); 
    }    

---

Edit

 public ExtensionAccount(ApexPages.StandardController controller) {
      this.acct = (Account)controller.getRecord();
    }
    private final Account acct;  

    // The constructor passes in the standard controller defined
    // in the markup below
    public ExtensionAccount(ApexPages.StandardSetController controller) {
        this.acct = (Account)controller.getRecord(); 
    }    


    public List<Task> getTaskList() {
         return [SELECT AccountId FROM Task Where WhatId IN (Select id From Opportunity Where AccountId = :acct.Id)];
    }  
}