I haven't managed to reproduce this with a simplified project, so it may or may not be related to this asset store asset...
Repro Steps:
- Download this project
- run the Clock scene
- in the Project window, search for "black" - number of results will show up
- alternate between the reach filters (All, In Packages, Asset Store, etc)
- at some point, you'll notice the filters text going red and this warning in the console:
Unable to find style 'ExposablePopupItem' in skin 'GameSkin' <called outside OnGUI>
UnityEditor.AsyncHTTPClient:Done(State, Int32)
Regression from 2018.3.0b3