I am save a record using pop window.. VF to popup(VF)

I got following error.. When i add a records using popup.. First time its working fine.. in second time got error


"The page you submitted was invalid for your session. Please click Save again to confirm your change"

public ID rowId {get;set;}
public incident_Item_Assets__c objForPage2{get;set;}
public Pagereference goToPage2(){
 
for(incident_Item_Assets__c obj : IncConfigsItemAsset){
            if(obj.id==rowId){
                objForPage2= obj;
        System.debug('test123'+objForPage2);
            }
        }
     return page.vfactivitypopup;
 }

public void activitysave(){
actasset.AssetID__c = ApexPages.currentPage().getParameters().get('id');
Insert actasset;
}


 page 1
  <apex:column >
<apex:commandLink action="{!goToPage2}"  value="popup" reRender="selected"  onclick="window.open('/apex/vfactivitypopup?id={!com1.id}','','width=550,height=450');" >
  <apex:param name="rowId" assignTo="{!rowId}" value="{!com1.id}"  />
</apex:commandLink>
</apex:column>
 page 2
  <apex:panelGrid columns="2">
          <apex:outputText value="Incident Item Asset"/>
          <apex:outputText style="font-weight:bold" value="{!objForPage2.id}"/>                    
        </apex:panelGrid>