Steps to reproduce(or just watch user's attached gif ):
1. Open my project "OpenMe".
2. Expand "Window" tab and select "SomeWindow".
--- For aesthetics you can attach this tab to other windows.
3. Save layout and open it.
4. Delete "SomeWindow" script in "Editor" folder.
--- It's important that before you delete this script, "SomeWindow" window should be opened. For reference watch attached photo.
5. Change layout to default.
6. Reopen saved layout.
--- You should experience a crash.

Workaround: When deleting "SomeWindow" script don't open this window.

Reproduced with: 5.4.0f2, 5.4.0b25, 5.4.0b22
Not reproduced with: 5.3.6f1, 5.4.0b21
Regression introduced in: 5.4.0b22