- Download the Inventory Pro (by DevDog) package from the Asset Store in Unity 5.6 (you will see a pop-up window with some information about this package, don't close this window)
- Close the project and reopen it in Unity 2017.1 to force an update
- Find any script in the project, right click on it in the Project Browser and select Reimport to force a script recompilation

Actual: the pop-up window renders all black and the following exception is printed in the console:

```NullReferenceException: Object reference not set to an instance of an object
at UnityEngine.Experimental.UIElements.VisualContainer.LoadStyleSheetsFromPaths () [0x00074] in C:\buildslave\unity\build\Runtime\UIElements\Managed\VisualContainer.cs:302
at UnityEditor.GUIView.get_panel () [0x0008b] in C:\buildslave\unity\build\Editor\Mono\GUIView.cs:43
at UnityEditor.GUIView.get_visualTree () [0x00002] in C:\buildslave\unity\build\Editor\Mono\GUIView.cs:53
at UnityEditor.DockArea.OnEnable () [0x0009c] in C:\buildslave\unity\build\Editor\Mono\GUI\DockArea.cs:120 ```

Expected: no errors and all editor windows still render correctly

This is a regression introduced in Unity 2017.1