How to reproduce:
1. Create a new Unity project
2. Select any Package manifest file(e.g. Packages/Unity UI/package.json)
3. Change the Inspector window from 'Normal' to 'Debug' mode
4. Change the Inspector window back from 'Debug' to 'Normal' mode

Expected result: no errors are thrown in the Console window
Actual result: an error is thrown in the Console window

Reproducible with: 2019.2.0a9, 2019.2.4f1, 2019.3.0b2, 2020.1.0a3
Not reproducible with: 2018.4.8f1, 2019.2.0a8
Could not test with: 2017.4.32f1(Package Manager not available)

Notes:
The following error is thrown:
PackageManifestImporterEditor.OnInspectorGUI must call ApplyRevertGUI to avoid unexpected behaviour.
UnityEditor.GenericMenu:CatchMenu(Object, String[], Int32) (at /Users/builduser/buildslave/unity/build/Editor/Mono/GUI/GenericMenu.cs:121)