Either of these would previously crash (same for VX):
tm = Tilemap.new
at = tm.autotiles
tm = nil
GC.start
at[0] = Bitmap.new(1, 1)
tm = Tilemap.new
at = tm.autotiles
tm.dispose
at[0] = Bitmap.new(1, 1)
Funnily, this makes RMXP itself crash too, but crashing is
never acceptable except for possibly resource exhaustion.