If the createlayersurface callback is hit when selmon is NULL then wlr_layer_surface is destroyed by calling wlr_layer_surface_v1_destroy(). This change simply adds a return at that point, to prevent a crash caused by dereferencing the destroyed variable.

The crash in question can be reproduced easily by unplugging or turning off all monitors while running swaync, waiting a few seconds and then plugging them back in. It is likely possible to cause this with things other than swaync, but that was the application that triggered it for me.