When importing a Unity Package with the "Import Unity Package" window open, if you click on an asset that can be previewed and then click on another asset that can be previewed, a NullReferenceException will occur.

NullReferenceException: Object reference not set to an instance of an object
UnityEditor.PopupWindow.OnGUI () (at C:/buildslave/unity/build/Editor/Mono/GUI/PopupWindow.cs:93)

1. Open Unity
2. Open the attached project
3. In the Project Browser, navigate to Packages > TextMesh Pro > Package Resources
4. Open TMP Examples & Extras.unitypackage
5. When the Import window appears, scroll down to the Textures folder
6. Click on a texture to preview it
7. Click on another texture to preview it

Expected Result:
The texture can be previewed and no errors appear in the console.

Actual Result:
The texture preview does not appear and a NullReferenceException occurs.

Tested on Windows.

Reproduced on 2019.1.0a9, 2019.1.0a10, 2019.1.0a12, 2019.1.0b1, 2019.1.0b6, 2019.2.0a8, 2018.3.7f1

Cannot reproduce on 2019.1.0a1, 2019.1.0a7, 2019.1.0a8