If you create a background and switch tabs away from it then often the graphics will be missing and switching back to the tab for the background will make Stencyl crash with the error popup. If you save immediately after loading up the graphics, before you do anything else like switch tabs, then that seems to prevent the problem.
Edit:
Actually I think I just had it crash even after saving, when I returned to that background tab. The background was still there though, and then after closing and reopening Stencyl, it crashed again when opening the background, but after that it seems to work fine.
Edit2:
It randomly crashed again when switching to a background that was working fine earlier in that session.
I'm using the public 4.0.4 build from the front page.
Edit:
Actually I think I just had it crash even after saving, when I returned to that background tab. The background was still there though, and then after closing and reopening Stencyl, it crashed again when opening the background, but after that it seems to work fine.
Edit2:
It randomly crashed again when switching to a background that was working fine earlier in that session.
I'm using the public 4.0.4 build from the front page.