How to reproduce:
1. Create a new project
2. Set the Layout to 'Default' (Window -> Layouts -> Default)
3. Inside the Assets directory create a subfolder 'Editor' and paste the attached Script (OpenFolderTest.cs) inside it
4. Switch from the Project tab to the Console tab
5. Close and reopen the Editor
6. Without switching to the Project tab click 'Open Test' -> 'Run Open Folder Test' located at the top of the Editor

Expected result: the 'Editor' folder is selected (selection can be seen in the Inspector window)
Actual result: NullReferenceException is thrown (but the selection can still be seen)

Reproducible with: 2019.1.0a12, 2019.4.9f1, 2020.1.4f1, 2020.2.0b1
Not reproducible with: 2018.4.27f1, 2019.1.0a11

Workaround:
After reopening the Editor at step 5 switch to the Project tab for a moment (it's not necessary to stay in the Project tab after that)

Notes:
The issue does not occur when the Project window is closed