Original Reproduction steps:
1. Open attached project "Galactic Wars.zip" and scene "GridArrayTest"
2. In Hierarchy window, select "Game_Manager" GameObject
3. In Inspector window, click on "Grid Config To Load" field's Object Picker
4. Observe Console window

Expected result: No error, Object Picker opens
Actual result: "NullReferenceException" error, Object Picker doesn't open

Reproducible with: 2019.4.16f1, 2020.1.17f1, 2020.2.0f1, 2021.1.0a9, 2021.2.0a1
Could not test with: 2018.4.30f1

Updated repro steps:
1- Copy the layout file galataic.wlt in <Users>/AppData\Roaming\Unity\Editor-5.x\Preferences\Layouts\default. This will make this layout available for any project
2- Open any project select the galatci layout
3- Click on an property field that should pop an ObjectSelector.
4- See error in console.