Using Action Filters on MVC C# using query String

Im using class name RightCheckerAttribute to check user permission in MVC3 application...
So the RightCheckerAttribute class is like this...

    public bool isAdmin { get; set; }
 
     public override void OnActionExecuting(ActionExecutingContext filterContext)
     {
         HttpContextBase context = filterContext.HttpContext;
 
         bool result = Convert.ToBoolean(context.Request.QueryString["isAdmin"].ToString());
 
         if (isAdmin != result) 
         {
             RouteValueDictionary redirecttargetDictionary = new RouteValueDictionary();
             redirecttargetDictionary.Add("action", "NoPermission");
             redirecttargetDictionary.Add("controller","Singer");
             filterContext.Result = new RedirectToRouteResult(redirecttargetDictionary);
 
         }
 
         //base.OnActionExecuting(filterContext);
     }
 

So in Method i applying this have head as..

[RightChecker (isAdmin=true)]

Im Executing this method as this..

http://localhost:5576/Singer/DeleteSinger?isAdmin=true

The problem is whether I'm passing true or false... I got result variable as false... And I'm getting:

Exception[Null Object references]...

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