The fillet reduces the stress concentration at the corner. In general it's bad to have sharp inside corners - this is why the lightening holes are round, or have rounded ends. If you forgot to include the fillets when cutting the centre sheeting, you can glue a triangular piece of sheet material in there. The curve will distribute the stress a bit more evenly, but a 45° fillet will be almost as good.

If something bad happens, it would be cracks radiating from the sharp corner.