I upgraded IntelliJ Idea from 12.0.4 to 12.10.
Now all the modules in my Android project give the error:
Error: Default Activity Not Found
@TouchBoarder almost had it. Although selecting "Do not launch Activity" results in nothing launching.
In Android Studio under Run/Debug Configuration -> Android Application -> General -> Activity -> select the option "Launch:"
Choose your Activity. This doesn't exactly fix the intended behaviour but rather overrides it correctly.