A Trigger which doesn't allow Opportunity Stage to be changed if there are open task on that opportunity's related list

trigger CheckOpenActivityHasTaskInit on Opportunity (after insert,after update) 
{
   Set<String> OppoId = new Set<String>();
   List<Opportunity> oppp= new List<Opportunity>();
   for(Opportunity Opp : Trigger.New)
   {    

         List<AggregateResult> tsk =

         [SELECT Count(Id)countTask FROM Task WHERE IsClosed=false            and WhatId =:opp.Id];

         Integer tskCount = (Integer)tsk[0].get('countTask');       

         if(tskCount > 0)

         {

               opp.addError('To Procede Further Please! Close The                  Open Activities First...');

          }
     } 
}