Combos should not be considered Valid until there is an output key in the UI. Also combos should not be considered valid unless there is 2

[ujl123]

This combo should not be considered valid if there is no output key selected. In addition because I chose Mouse 1 it disabled my mouse's left click and took me a few mins to understand what was happening.

This combo should probably not be considered valid as well. The common understanding of a combo that it is a chord of 2 or more keys to create a new output. I don't really see a need for a 1 key combo, but perhaps someone could answer that one

Proposal
If in the GUI someone tries to do an invalid combo, warn and block it.