That door looks like the one on our Sunstar.
If you look at this thread:
You can see pictures of the door construction. Specifically,
The door is a sandwich of the outer skin, foam, and that thin plastic inner skin, wrapped in an aluminum extrusion frame.
Since the cracks are on each side of the window, I would remove the screws around the inside window bezel, remove it, and see if you can glean any more information.
It could be that the inner skin is delaminating from the foam, perhaps due to temperature extremes or poor assembly.
If so, you might be able to remove the door, lay it flat, inject some glue into the cracks, cover them with something non-stick like parchment paper, and put some weight on them to stick them back down.
If you remove the bezel, please post some more pictures.