thread.cs not found in C#

Recently I received an error in Visual Studio 2015 like this..

TargetInvocationException was not handled
An unhandled exception of type 'System.Reflection.TargetInvocationException' occurred in PresentationFramework.dll

In addition, the main window stated thread.cs not found.   Because I made a few changes before running the code (it compiled fine)


It took me a while to locate the problem, it linked back to a XAML control.
I previously had the code...

GetRogue_Button.Visibility = Visibility.Hidden;

But, when the code ran, the Button itself was null.  The method I placed the hide code in was called through an TextChangedEventHandler (I also used an observable collection in my code). 

The solution:

if ( GetRogue_Button != null
     && GetRogue_Button.Visibility != Visibility.Hidden)
   {
      GetRogue_Button.Visibility = Visibility.Hidden;
   }