Ooooh, my first bug report.
In the campaign, in the battle at the supply depot (where you have an engineer who needs to build a bridge across a river), my engineer got killed (I believe by the ongoing burn from the bandit pyrokineticist? He had a tiny sliver of health left and then died at the start of my turn) and produced a little pop-up text message saying he was leaving the battle. After this, however, there was a little white arrow in his former space, and hovering over it gave the engineer's name and a "level 0 (null)" status, wih 0 hit points. When I was using my healer, I accidentally clicked on this null unit to heal (trying to heal the unit above it). This caused the game to freeze and be entirely unresponsive. Clicking on units did nothing and keyboard shortcuts also didn't have any visible effect.
Edit: this is with the most recent alpha build, I downloaded it earlier today.