Steps to reproduce:
1. Open the attached project
2. Select the Directional Light game object
3. Open up the Culling Mask drop-down (this project contains an Editor script that overrides the normal Light Editor)
4. Click anywhere in the Scene/Hierarchy while the drop-down is active
5. Observe the error in the Console

Note: this might not reproduce on every setup - I had the most success with Mac computers, but we've managed to reproduce it on Windows as well

Affected versions: 2020.1.0a22, 2020.1.3f1, 2020.2.0a21
Unaffected versions: 2018.4.26f1, 2019.4.8f1, 2020.1.0a21

Expected result: no exceptions; the PopupWindowContent does not call its OnGUI after the parent window is gone
Actual result: NullReferenceException: SerializedObject of SerializedProperty has been Disposed