Error:java.lang.NullPointerException (no error message)

delete .gradle file and after that 

Build > Clean Project

Build > Rebuild Project

try to run it, if it doesnt work then go to

File > Invalid Caches / Restart and select "Invalidate and Restart"

then close the project, reopen it and run it again.

參考：

http://stackoverflow.com/questions/36720798/gradle-hangs-on-executing-tasks-androidgeneratedebugsources