How to reproduce:

1. Open attached project
2. Open scene test.unity
- Note that an error appears in the console:

InvalidOperationException: The following game object is invoking the DontDestroyOnLoad method: BaseLoader. Notice that DontDestroyOnLoad can only be used in play mode and, as such, cannot be part of an editor script.
testScript.OnEnable () (at Assets/testScript.cs:11)

- Reproduced in Version 5.4.0b11 (323b0238353b), Version 5.4.0b7 (83df6fa5e23f), Version 5.3.4f1 (fdbb5133b820)
- Not reproducible in Version 5.4.0b6 (2c3d6b93e789), Version 5.2.4f1 (98095704e6fe)

- By Design: DontDestroyOnLoad was never intended to work in edit mode. Only in play mode and standalone players.
- This is why it now throws an exception if it is used in edit mode