Ah, I see. I misunderstood the trouble you were having. The reason you are seeing this error is because the default game mode in the config files of a fresh ATBTT project is BP_ATBTT. BP_ATBTT has an event begin play event that looks for the grid manager. To stop this error message from occuring modify BP_ATBTT so it does not run all its EventBeginPlay nodes when it spawns, but rather from a custom event you can trigger when you want.