Usually that kind of error appears when AW tries to restart on top of a running (or more likely shutting down) instance - I think you can kind of force it by restarting from the menu quickly because the shutdown might be faster than the creation of a tile involving many Cu clouds, so by the time the tile is done the tile indexing structure is already removed, leading to the error - which is harmless because a new tile generation call with a new indexing structure is already on the way.
I'm at a loss how it would appear 'like that'...