This is my controller here i got the error system null pointer exception. any body give your suggestion how to solve this error.


public with sharing class AssignWorklistController

{
string subject;
        string description;
        string EmailList;

    public static Events__c Event {
        get {
            if (Event == Null || Event !=Null)
            {
               //String EventId  =ApexPages.currentPage().getParameters().get('EventId');
               //if(ApexPages.currentPage().getParameters().get('EventId') != null){
                 String EventId =String.escapeSingleQuotes(EncodingUtil.urlDecode(ApexPages.currentPage().getParameters().get('EventId'), 'UTF-8'));
                            
                if(EventId != '' && EventId != null )
                {
                  try
                   {
                     Event=[Select Events__c.Name, Assigned_to__c,Event_Type__r.CatalogType__c, Assigned_to__r.Name, Description__c  From Events__c  WHERE  Id=:EventId limit 1];
                     Event.Assigned_To__c=null;
                     //Event.Description__c=Null;
                   }
                
                  catch(exception e)
                  {
                   ApexPages.addMessages(e);
                  }
               }
               //}
            }
           
            return Event;
           
        }
       
        set;
    }
   
    public AssignWorklistController ()
    {
    }

    public PageReference AssignEvent ()
    {
        if(Event!=null)
        {
                
            try {
              
               Update Event;
               system.debug('Event Updated==');
               system.debug('Event Updated=='+Event.Assigned_to__c);
              // User u = [select Id,email from User where Id=: Event.Assigned_to__c];
               Staff__c s =[select Id, Email_Id__c from Staff__c  where Relatedstaff__c =:Event.Assigned_to__c];
               system.debug('s.Email======'+s.Email_Id__c );
               system.debug('s.Id======'+s.Id);
               system.debug('UserInfo.getUserEmail()======'+UserInfo.getUserEmail());
               if(s.Email_Id__c != null){
               EmailList ='email-list\nsagarcloudcrm@gmail.com\n'+UserInfo.getUserEmail()+'\n'+s.Email_Id__c;
                             
               system.debug('EmailList=='+EmailList);
                subject = Event.Event_Type__r.CatalogType__c;
                      subject = subject+string.valueof(system.now());
               system.debug('subject =='+subject );
               if(Event.End_Date_Time__c != null){
               description ='You had Assigned with an Event Named '+Event.Event_Type__r.CatalogType__c+'  '+'Your End Date on Task is on'+Event.End_Date_Time__c ;
             }else{
                 description ='You had Assigned with an Event Named '+Event.Event_Type__r.CatalogType__c;
             }
             // string Response = VerticalResponseAPIClient.SendVREmail(subject, EmailList, subject, UserInfo.getUserEmail(), description);
             //  system.debug('Response ==='+Response );
                 }
               System.debug('vent.Assigned_to__r.Name========'+Event.Assigned_to__r.Name);
               if(ApexPages.currentPage().getParameters().get('EventId') != null){
             
              String EventId =String.escapeSingleQuotes(EncodingUtil.urlDecode(ApexPages.currentPage().getParameters().get('EventId'), 'UTF-8'));

                if(EventId != '' && EventId != null)
                {
                 Events__c E = [Select Events__c.Name, Assigned_to__c,Event_Type__r.CatalogType__c, Assigned_to__r.Name, Description__c  From Events__c  WHERE  Id=:EventId limit 1];
                 System.debug('vent.Assigned_to__r.Name========'+E.Assigned_to__r.Name);  
               Task_List__c TL = new Task_List__c();
               TL.Assigned_To__c = E.Assigned_to__r.Name;
               TL.Created_By__c = UserInfo.getName();
               TL.Description__c = E.Description__c;
               TL.Status__c ='Pending';
               TL.Subject__c = E.Event_Type__r.CatalogType__c ;
               TL.Assigned_Date__c = DateTime.Now();
               TL.UniqueKey__c = String.valueof(TL.Assigned_To__c+TL.Created_By__c +TL.Assigned_Date__c);
             
               Insert TL;
              
               Event.Task_UniqueKey__c = TL.UniqueKey__c;
               Update Event;
                 System.debug('vent.Assigned_to__r.Name========'+E.Assigned_to__r.Name);
           
               }
              }
           }
           catch (Exception e)
            {
                ApexPages.addMessages(e);
            } 
             
        }   
       
         //SendMail();
       return null;
      
    }
   
     public PageReference SendMail()
    {
   
    system.debug('subject===='+subject);
    system.debug('EmailList===='+EmailList);
    system.debug('description===='+description);
    if(subject != null && EmailList!=null && description != null){
   
     string Response = VerticalResponseAPIClient.SendVREmail(subject, EmailList, subject, UserInfo.getUserEmail(), description);
               system.debug('Response ==='+Response );
               }
     PageReference pageRef = new PageReference('/apex/WorkListView');
        return pageRef;
    }
    public PageReference Cancel()
    {
        PageReference pageRef = new PageReference('/apex/WorkListView');
        return pageRef;
    }
}


Advance Thanks
Maheswar