Visualforce Error: "Return type of an Apex action method must be a PageReference. Found: core.apexpages.el.adapters.ApexMapELAdapter"

Question

This is a follow-up question to: When attempting to use a Map I get "System.NullPointerException: Attempt to de-reference a null object"

I am now receiving "Return type of an Apex action method must be a PageReference. Found: core.apexpages.el.adapters.ApexMapELAdapter " when trying to return a Map value with a Integer and custom object to a Visualforce Page.

Visualforce Page:

<apex:page standardController="Opportunity_Package__c" extensions="MetadataPackageEditor" action="{!getPackageMap}">
  <apex:form >
      <apex:inputField value="{!Opportunity_Package__c.Opp_Package_Rel__c}" id="Opporunity"/>
      <apex:pageBlock title="Select Package(s)">
      <apex:pageMessages />
      <span>Name </span>
      <apex:inputField value="{!Opportunity_Package__c.Name}"/>

      <table style="width:100%;">
          <tbody>
              <tr>
                  <th>Select</th>
                  <th>Package</th>
                  <th>Products</th>
                  <th>PPU Discount</th>
                  <th>Expiration Date</th>
              </tr>

              <apex:repeat value="{!packageMap}" var="oppPackage">
              <tr>
                  <td></td>
                  <td><apex:inputField value="{!packageMap[oppPackage].Name}" /></td>
                  <td><apex:inputField value="{!packageMap[oppPackage].Products__c}" /></td>
                  <td><apex:inputField value="{!packageMap[oppPackage].PPU_Discount__c}" /></td>
                  <td></td>
              </tr>
              </apex:repeat>
          </tbody>
      </table>

      </apex:pageBlock>
  </apex:form>
</apex:page>

Code causing the issue:

Map<Integer, Package__c> packageMap { get; set; }


public Map<Integer, Package__c> getPackageMap() {

    Integer i = 0;

    packageMap = new Map<Integer, Package__c>();

    for (Package__c pack : [SELECT Name, Products__c, PPU_Discount__c FROM Package__c]) {
        packageMap.put(i, pack);
        i++;
        }

    return packageMap;

}

Opportunity Package is a junction record between Opportunity, and my custom object Package. Package is pulled via lookup, and then queried for it's detailed information.

Answer 1

On line 1, you have an action set to {!getPackageMap}. An action function must return "void" or "null" (no value), or a valid PageReference. It cannot return any other data type, or you'll get an error message.

Since you're actually calling that function from apex:repeat, there's no reason why you need to call the action method. You should remove it from your markup on line 1.

<apex:page standardController="Opportunity_Package__c" extensions="MetadataPackageEditor" <!-- action="{!getPackageMap}" --> >