I'm getting the error "Return type of an Apex action method must be a PageReference. Found: core.apexpages.el.ApexELResolver$VisualforceArrayList"

What is the issue?

/**Visual page **/

<apex:page standardController="Applicant__c" recordSetVar="applicant" extensions="view">
  <apex:form >
      <apex:pageBlock >
          <apex:outputLabel value="Status :" for="status"/>
          <apex:inputField id="status" value="{!Applicant__c.Status__c}">
          <apex:actionSupport event="onchange" action="{!getApplicantByStatus}" reRender="view"/>
              <apex:param name="para" value="{!Applicant__c.Status__c}" assignTo="{!para}"/>
             
            
          </apex:inputField>
      
      </apex:pageBlock>
       <apex:pageBlock >
           <apex:panelgroup id="view">
              <apex:pageBlockTable id="viewApplicant" value="{!applicantByStatus}" var="app">
                  <apex:column headerValue="Applicant ID" value="{!app.Name}">
                              
                  </apex:column>
                  <!-- <apex:column headerValue="Applicant Name" value="{!app.Name__c}">
              
                  </apex:column>-->
              </apex:pageBlockTable>
           </apex:panelgroup>
      </apex:pageBlock> 
      
  </apex:form>
</apex:page>

 /**Controller code**/

public class view {

    public Applicant__c applicant;
    public List<Applicant__c> applicantByStatus {get;set;}
    private ApexPages.StandardSetController controller;
   
    public List<String> applicantNameByStatus{get;set;}
    
    public String para{
        get;
        
        set{
           para=value;
        
        }
    }
    
    public List<String> displayFields {
        get; private set;
    }
    
    public view(ApexPages.StandardSetController controller) {
        this.applicant=(Applicant__c)controller.getRecord();
        //this.controller = controller;
    }
    
    public List<Applicant__C> getApplicantByStatus(){
        
         applicantByStatus=new List<Applicant__c>();
         applicantByStatus=[SELECT Name,Name__c,Status__c,Interview_Date_Time__c FROM Applicant__c WHERE Status__c =:para];
         
         if(applicantByStatus==null){
             System.Debug('APPLICANT STATUS IS NULL');
         }
         for(Applicant__c app:applicantByStatus){
             String name=app.Name__c;
             System.Debug('CHECK :: '+app.Name__c);
             applicantNameByStatus.add('app.Name__c');
         }
         
         //applicantNameByStatus.add('Chamil');
          //applicantNameByStatus.add('madusanka');
          // applicantNameByStatus.add('applicant1');
         
         return applicantByStatus;
    } 
}