Thanks for reporting!
I managed to reproduce the problem in Campaign mode by removing the weapon from the first soldiers hand. The issue happens when the UI tries to check the reaction fire mode for the soldier who has no weapon in hand. Apparently we patched the code in 2013 already, but it wasn't good enough.
I got it fixed in 9cad041f0931b1f49e0c70f32e351a7227f0d2ff in branch ufoai_2.5. I'm going to check if the master branch is affected.
Thanks again!
-geever