Try to clean & then Rebuild the project again. To do so please go to Build menu, both the tools are present their. If issue not solved with that than try to invalidate caches & restart Android Studio from File menu -> Invalidate Caches/Restart.
We can manage it by changing one method of GMUDefaultClusterIconGenerator class. In GMUDefaultClusterIconGenerator.m replace below method, with What i have done is, I have just change else portion and set text as exact number instead of string with +!