IndexNotReadyException - Android Studio
Happens because of the background indexing processes.
When the IDE is indexing, if we try to change the values in properties tab, then IndexNotReadyException is thrown.
Wait till indexing is complete.
Hope Google fixes this in AndroidStudio's next release!
File-> Invalidate Cache and Restart
This will stop the old indexing process and start a new one.
After that wait for indexing, and let that project build.
I faced the same problem and found solution according to this, I have changed the API level 22 to 21, problem solved.
API Level for XML layout was set **Automatically Pick Best ** which was API 22
And I change this API 22 to API 21:
and the exception removed.
Hope this help. Thanks