Fix DestroyPlatformWindow crashing when called in Shutdown of the application

Juliet/src/Core/HAL/Display/Display.cpp

@@ -53,7 +53,10 @@ namespace Juliet
         }
 
         // Destroy all Windows that are still alive
-        DestroyPlatformWindow(CurrentDisplayDevice->MainWindow);
+        if (CurrentDisplayDevice->MainWindow)
+        {
+            DestroyPlatformWindow(CurrentDisplayDevice->MainWindow);
+        }
 
         CurrentDisplayDevice->Shutdown(CurrentDisplayDevice);
         // Free anything that was freed by the shutdown and then free the display
@@ -89,11 +92,14 @@ namespace Juliet
 
     void DestroyPlatformWindow(NonNullPtr<Window> window)
     {
+        Assert(CurrentDisplayDevice->MainWindow == window.Get());
+
         HideWindow(window);
 
         CurrentDisplayDevice->DestroyPlatformWindow(CurrentDisplayDevice, window);
 
         Free(window.Get());
+        CurrentDisplayDevice->MainWindow = nullptr;
     }
 
     void ShowWindow(NonNullPtr<Window> window)