Hi I have a Controller that dynamically adds Rows and new records through a Visual Force page. I had the page working perfectly as a detail button, however when I added in the elements to make it a list button, I recieve the following error on  Add Row execution:

Attempt to de-reference a null object
Error is in expression '{!addRows}' in component <apex:commandButton> in page resource_allocation: Class.ManageListController.addRows: line 46, column 1

Not sure what has changed since adding the List Button lines as the liones below haven't altered. Any help would be greatly appreciated!

Controller
public class ManageListController 
{
   private final Resource_Allocation__c RA;
     // The extension constructor initializes the private member
        // variable RA by using the getRecord method from the standard
        // controller.
        public ManageListController(ApexPages.StandardSetController stdController) {
            this.RA = (Resource_Allocation__c)stdController.getRecord();
        }

 public List<ResourceAllocationWrapper> wrappers {get; set;}
 public static Integer toDelIdent {get; set;}
 public static Integer addCount {get; set;}
 private Integer nextIdent=0;
  
 public ManageListController()
 {
  wrappers=new List<ResourceAllocationWrapper>();
  for (Integer idx=0; idx<1; idx++)
  {
   wrappers.add(new ResourceAllocationWrapper(nextIdent++));
  }
 }
  
 public void delWrapper()
 {
  Integer toDelPos=-1;
  for (Integer idx=0; idx<wrappers.size(); idx++)
  {
   if (wrappers[idx].ident==toDelIdent)
   {
    toDelPos=idx;
   }
  }
   
  if (-1!=toDelPos)
  {
   wrappers.remove(toDelPos);
  }
 }
  
 public void addRows()
 {
  for (Integer idx=0; idx<addCount; idx++)
  {
   wrappers.add(new ResourceAllocationWrapper(nextIdent++));
  }
 }
  
 public PageReference save()
 {
  List<Resource_Allocation__c> RA=new List<Resource_Allocation__c>();
  for (ResourceAllocationWrapper wrap : wrappers)
  {
   RA.add(wrap.RA);
  }
   
  insert RA;
   
  return new PageReference('/' + Schema.getGlobalDescribe().get('Opportunity').getDescribe().getKeyPrefix() + '/o');
 }
  
 public class ResourceAllocationWrapper
 {
  public Resource_Allocation__c RA {get; private set;}
  public Integer ident {get; private set;}
   
  public ResourceAllocationWrapper(Integer inIdent)
  {
   ident=inIdent;
   RA=new Resource_Allocation__c(Name='RA' + ident);
  }
 }
}

VisualForce Pagew
<apex:page standardcontroller="Resource_Allocation__c" recordSetVar="Resource Allocations" tabstyle="Resource_Allocation__c" extensions="ManageListController" >
<apex:form >
   <apex:pageBlock title="Bulk Resource Allocation Create">
      <apex:pageBlockTable value="{!wrappers}" var="wrapper" id="wtable">
         <apex:column headerValue="Ident">
            <apex:outputText value="{!wrapper.ident}"/>
         </apex:column>
        <apex:column headervalue="Opportunity">
        <apex:inputField value="{!wrapper.RA.Opportunity__c}"/>
        </apex:column>
        <apex:column headerValue="Resource Name or Role">
         <apex:inputField value="{!wrapper.RA.Resource_Name_or_Role__c}"/>
                 </apex:column>
        <apex:column headerValue="Start">
            <apex:inputField value="{!wrapper.RA.Start_Date__c}"/>
        </apex:column>
         <apex:column headerValue="End">
            <apex:inputField value="{!wrapper.RA.End_Date__c}"/>
         </apex:column>
         <apex:column headerValue="Est Pay">
            <apex:inputField value="{!wrapper.RA.Resource_Est_Pay__c}"/>
         </apex:column>
         <apex:column headerValue="Est Charge">
            <apex:inputField value="{!wrapper.RA.Resource_Est_Charge__c}"/>
         </apex:column>
        <apex:column headerValue="Max Days Contracted">
        <apex:inputfield value="{!wrapper.RA.Max_Days_Contracted__c}"/>
               </apex:column>
         <apex:column headervalue="Daily Expense Allowance">
               <apex:inputField value=" {!wrapper.RA.Daily_Expense_Allowance__c}"/>
               </apex:column>
         <apex:column headerValue="Action">
            <apex:commandButton value="Delete" action="{!delWrapper}" rerender="wtable">
               <apex:param name="toDelIdent" value="{!wrapper.ident}" assignTo="{!toDelIdent}"/>
            </apex:commandButton>
         </apex:column>
      </apex:pageBlockTable>
      <apex:commandButton value="Add Row" action="{!addRows}" rerender="wtable">
         <apex:param name="addCount" value="1" assignTo="{!addCount}"/>
      </apex:commandButton>
      <apex:commandButton value="Add 5 Rows" action="{!addRows}" rerender="wtable">
         <apex:param name="addCount" value="5" assignTo="{!addCount}"/>
      </apex:commandButton>
      <apex:commandButton value="Save" action="{!save}"/>
   </apex:pageBlock>
 </apex:form>
</apex:page>

(Code credit to http://bobbuzzard.blogspot.com/2011/07/managing-list-of-new-records-in.html)