How to reproduce:
1. Open attached project "Case1256213_reloadWindow.rar"
2. Select "SimpleMaterial" Material
3. Click on the triple dot button next to the Material preview's header at the bottom of the Inspector
4. Select "Convert to Floating Window"
5. Click on the triple dot button on the newly opened window's header
6. Select "Reload Window"

Expected result: Editor does not crash
Actual result: Editor crashes

Reproducible with - 2020.1.0a22, 2020.1.0b16, 2020.2.0a18
Not reproducible with - 2020.1.0a21

First 4 lines of stack trace:
ActiveEditorTracker_CUSTOM_Internal_GetActiveEditorsNonAlloc
UnityEditor.ActiveEditorTracker:Internal_GetActiveEditorsNonAlloc (UnityEditor.ActiveEditorTracker,UnityEditor.Editor[])
UnityEditor.PropertyEditor:Update ()
UnityEditor.HostView:SendUpdate ()