Bugfix: Calling ApplyChanges() in Game actor cause Exception

Reported as MonoGame#5173 (MonoGame#5173)

This is a temp fix. Calling ApplyChanges() should create the graphics
Device anytime.

MonoGame.Framework/GraphicsDeviceManager.cs

@@ -416,6 +416,11 @@ private void CreateDevice_SDLNew()
         /// </summary>
         public void ApplyChanges()
         {
+#if WINDOWS_UAP
+            if (_graphicsDevice == null)
+                return;
+#endif
+
             // If the device hasn't been created then create it now.
             if (_strategy.GraphicsDevice == null)
                 CreateDevice();