The editor will crash if you try adding an interface to an actor that already has a variable with the same name as one of the functions in the interface. See the attached sample project as an example. Add MyInterface to MyActor and then click Compile on the actor to crash the editor.
Thank you for reporting this issue. It seems that this is a regression from 4.16 to 4.17. In 4.16, an error was returned for doing this rather than the editor crashing. I’ve entered a bug report for the issue which can be found here: UE-51260
Have a nice day!