Are you freeing the tilemaps? (Again this comes down to how you have structured them) If they point to each other (Internsly or externaly) them even tho the mian object goes out of scope, the internal refrences would stop the reference pointer from zeroing.
But Im with Tonyg on this, try to make the smalest code possible that still has the problem.
But Im with Tonyg on this, try to make the smalest code possible that still has the problem.