I hate null reference errors. I am glad you figured out what the problem was. Looking forward for the next release and thank you very much.
P.S. I have a little additional request for interface popup sounds. My UFO alarm sound is played every time I click on the UFO to look at it for informations, but this is a bit weird. Looking at UFOs at the globe I am hearing for each one a alarm signal and "Warning, new enemy contact detected!" voiceover. Can you maybe implement a flag to only play the sound once for the first time a new UFO get spotted and the event window pops up? My original intention for the UFO event window sound was to only warn of new contacts on detection.