Steps to reproduce:

1. Open attached project
2. Open scene "multiple buttons event test"
3. Add one more Inspector window. Now, there should be two in total
4. Make sure that both of these windows are visible
5. In hierarchy, expand "Canvas" and select multiple buttons
6. In inspector, Press "+" to add one more OnClick() event
7. Notice NullReferenceException thrown in console

Note: this also happens when adding events to other UI components (e.g. toggle component)

Workaround: use only one Inspector window