“Default Activity Not Found” on Android Studio upgrade

Question

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

Answer 1

Sync Project With Gradle Files works sometimes, To fix this Overall issue you should

1. Exit Android Studio
2. Go to USER -> AndroidStudio -> system -> caches
3. Delete that folder
4. Start Android Studio.

It will re-index your files and that should work

Thanks to kirtan403 from a similar question.