I faced this problem in Android studio that said "Render problem- Failed to load AppCompat ActionBar with unknown error." I looked up for the solution online and found this "Android studios 3.1 does not render xml file due to java class missing" link. I tried the second method and it worked. Even thought the problem is resolved I'd still like to know why did this problem occur and what changes did the addition of "Base." make that the problem was solved.