After updating the Android SDK, I started Android Studio. Now its showing background task continuously running with message
Updating indices: Scanning files to index.
In this mode, it is nearly hanged and I can not work on it. Not able to find a way to stop background task. A restart of Android Studio didn't work either.
I've find out that I should use the solution of @free3dom which is the Invalidate Caches / Restart option in the File menu.
I have to do this every time I change some project file by switching branches in GIT.
And yes, Android Studio really sucks and is full of bugs...