Repro Steps:
- Open this project or start your own project
- I have the Tile Palette and the console window tabs selected, which causes the Hierachy and Project windows to not be visible
- Open a script in a different window
- Write up some errors in that script that causes Unity to fail the script to compile
- alt+tab to that scripting wondo again to fix the compile errors
- alt+tab to the Unity window to trigger the script to compile again
- alt+tab to a different window again while the scripts are compileing
- alt+tab to the Unity window and notice the hierechy window and the Project window have red text and this error appears in the Console:
Unable to use a named GUIStyle without a current skin. Most likely you need to move your GUIStyle initialization code to OnGUI
UnityEditor.SceneHierarchyWindow:OnEnable()

Note: there are probably smarter ways to get the UnityEditor.SceneHierarchyWindow:OnEnable() to trigger, but this is the only way I managed to do it myself. Reproduced this about 3-4 times in 10 mins, but I'm still not 100% sure what triggers this call.

Reproducible with Unity 2018.3.0b1
Not reproducible with Unity 2018.3.0a11 or earlier versions.