NullReferenceException while accessing ViewBag in Asp.net MVC4

Question

I am trying to access the ViewBag data in my view as below:

<span class="small">@ViewBag.BreadCrumb</span>

I am sending that ViewBag data from code like:

ViewBag.BreadCrumb = topic.Category.CatName + " / " + topic.Name;
ViewBag.TopicID = id;

Here, topic is the Entity. And I am returning the View using:

return View(topic);

But, it always send me an exception:

Object reference not set to an instance of an object.

In another page it's working very well. Is there any solution?

Edit::

Code of Action is as follows:

Topic topic = db.Topics.FirstOrDefault(t => t.TopicID == id);
ViewBag.Topics = from t in db.Topics where (t.CatID == topic.CatID) select t;
ViewBag.TopicID = id;
ViewBag.BreadCrumb = topic.Category.CatName + " / " + topic.Name;
return View(topic);

Edit::

Its shows following exception:

An exception of type 'System.NullReferenceException' occurred in App_Web_ieekpj10.dll but was not handled in user code

Additional information: Object reference not set to an instance of an object.

But in in Immediate windows it shows output: ViewBag.BreadCrumb "C Sharp Edit / Name- edited to C 12"

Edit:: Stack Trace

[NullReferenceException: Object reference not set to an instance of an object.]
ASP._Page_Views_Category_NewSection_cshtml.Execute() in e:\OutSourcingStuffs   \OnlineLibrary\src\OnlineLinkLibrary\OnlineLinkLibrary.Web\Views\Category\NewSection.cshtml:12
System.Web.WebPages.WebPageBase.ExecutePageHierarchy() +271
System.Web.Mvc.WebViewPage.ExecutePageHierarchy() +121
System.Web.WebPages.StartPage.RunPage() +63
System.Web.WebPages.StartPage.ExecutePageHierarchy() +100
System.Web.WebPages.WebPageBase.ExecutePageHierarchy(WebPageContext pageContext,   TextWriter writer, WebPageRenderingBase startPage) +177
System.Web.Mvc.RazorView.RenderView(ViewContext viewContext, TextWriter writer, Object instance) +762
System.Web.Mvc.BuildManagerCompiledView.Render(ViewContext viewContext, TextWriter writer) +382
 System.Web.Mvc.ViewResultBase.ExecuteResult(ControllerContext context) +431
System.Web.Mvc.ControllerActionInvoker.InvokeActionResult(ControllerContext controllerContext, ActionResult actionResult) +39
System.Web.Mvc.<>c__DisplayClass1a.<InvokeActionResultWithFilters>b__17() +74
System.Web.Mvc.ControllerActionInvoker.InvokeActionResultFilter(IResultFilter filter, ResultExecutingContext preContext, Func`1 continuation) +388
System.Web.Mvc.<>c__DisplayClass1c.<InvokeActionResultWithFilters>b__19() +72
System.Web.Mvc.ControllerActionInvoker.InvokeActionResultWithFilters(ControllerContext controllerContext, IList`1 filters, ActionResult actionResult) +303
System.Web.Mvc.Async.<>c__DisplayClass2a.<BeginInvokeAction>b__20() +155
System.Web.Mvc.Async.<>c__DisplayClass25.<BeginInvokeAction>b__22(IAsyncResult asyncResult) +184
System.Web.Mvc.Async.WrappedAsyncResult`1.End() +136
System.Web.Mvc.Async.AsyncResultWrapper.End(IAsyncResult asyncResult, Object tag) +56
System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeAction(IAsyncResult asyncResult) +66
System.Web.Mvc.<>c__DisplayClass1d.<BeginExecuteCore>b__18(IAsyncResult asyncResult) +40
System.Web.Mvc.Async.<>c__DisplayClass4.<MakeVoidDelegate>b__3(IAsyncResult ar) +47
System.Web.Mvc.Async.WrappedAsyncResult`1.End() +151
System.Web.Mvc.Async.AsyncResultWrapper.End(IAsyncResult asyncResult, Object tag) +59
System.Web.Mvc.Async.AsyncResultWrapper.End(IAsyncResult asyncResult, Object tag) +40
System.Web.Mvc.Controller.EndExecuteCore(IAsyncResult asyncResult) +68
System.Web.Mvc.Async.<>c__DisplayClass4.<MakeVoidDelegate>b__3(IAsyncResult ar) +47
System.Web.Mvc.Async.WrappedAsyncResult`1.End() +151
System.Web.Mvc.Async.AsyncResultWrapper.End(IAsyncResult asyncResult, Object tag) +59
System.Web.Mvc.Async.AsyncResultWrapper.End(IAsyncResult asyncResult, Object tag) +40
System.Web.Mvc.Controller.EndExecute(IAsyncResult asyncResult) +65
  System.Web.Mvc.Controller.System.Web.Mvc.Async.IAsyncController.EndExecute(IAsyncResult asyncResult) +39
System.Web.Mvc.<>c__DisplayClass8.<BeginProcessRequest>b__3(IAsyncResult asyncResult) +45
System.Web.Mvc.Async.<>c__DisplayClass4.<MakeVoidDelegate>b__3(IAsyncResult ar) +47
System.Web.Mvc.Async.WrappedAsyncResult`1.End() +151
System.Web.Mvc.Async.AsyncResultWrapper.End(IAsyncResult asyncResult, Object tag) +59
System.Web.Mvc.Async.AsyncResultWrapper.End(IAsyncResult asyncResult, Object tag) +40
System.Web.Mvc.MvcHandler.EndProcessRequest(IAsyncResult asyncResult) +66
System.Web.Mvc.MvcHandler.System.Web.IHttpAsyncHandler.EndProcessRequest(IAsyncResult result) +38
System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +9688704
System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +155

Edit: View

@model OnlineLinkLibrary.Models.Section
@{
   ViewBag.Title = "New Section";
}

<div class="panel panel-default">
<div class="panel-heading ">
    <div class="row ">
        <div class="col-xs-6">
            <h4>
                New Section
                <span class="small">@ViewContext.Controller.ViewBag.BreadCrumb</span>
        </h4>
    </div>
    <div class="col-xs-6 ">
        <div class="pull-right"> 
          -- satic htmls are there
        </div>
    </div>
</div>
</div>

Answer 1

If you are sure by 100% that the ViewBag is not null however Visual Studio keeps pointing to that line as null,

then look at the next line of code for the error, the compiler sometimes get confused ;)

This might help a drowning dev.

Answer 2

Your view is expecting a OnlineLinkLibrary.Models.Section:

@model OnlineLinkLibrary.Models.Section

But you provide a Topic. Unless Topic is a subclass of OnlineLinkLibrary.Models.Section, it wouldn't work.

Answer 3

I would suggest check if topic is null or not.

Topic topic = db.Topics.Where(a=>a.TopicID == id).FirstOrDefault();
if(topic != null)
{
  ViewBag.Topics = from t in db.Topics where (t.CatID == topic.CatID) select t;
}