Its a simple problem you are probably trying to create/upload a texture after device loss but before it was recreated.
I handled this in Genome2D as it is surprisingly quite often when people create textures even when the application is in the background or on some kind of event/timer where you can't know if there is device loss at the moment or not. So this must be handled by Starling at core to postpone initialization/upload of context texture till its possible.
Or you need to manage it at your side to avoid this when there is device loss which can be tricky.