It seems as though the delegate has not been initialized, but I am unsure why.
If you’re binding the delegate in the start up module of your plugin, then you can solve the issue by loading your plugin module later. Go to the uplugin file of your plugin and change the LoadingPhase to PostEngineInit. This change let me bind the OnLevelActorDeleted delegate in my start up module function. Without this change I’d also get an error with InvocationListLockCount.