To Reproduce:

1.Open the attached project, if scene 'emptyScene' is loaded load another scene, then reload 'emptyScene' & play.

2. Notice, the null reference error, from inside 'TestSMB.OnEnable', its thrown every time you run the scene.

3. Close Unity & reopen the project if 'emptyScene' is loaded the error is not thrown when playing it, but if you load it in editor, like in step 1, it's thrown every time you hit play. It happens relatively inconsistently, most of the times it's no longer called if you had entered play mode in 'test' scene, you have to reopen Unity to reproduce it.

Related to 631119.