DOSBox crashes after exiting the Mapper when you delete the Mapper key (Ctrl-F1 by default). The workaround for this is to save the keymap before exiting the Mapper and then restart DOSBox. Of course you have to subsequently modify or delete the keymap file if you only wanted to disable the Mapper key temporarily.

The crash happens because the key event object that runs the Mapper is needed as a return point, but the Mapper can delete it like any other key event object. I thought about putting a delete flag on the objects and postponing deletion, but it was turning out to be tricky, and a fair amount of code. The attached patch is a much simpler approach, and appears to be effective in my tests.