If I run gradle assembleDebug from the command line, I am suddenly getting this error:
gradle assembleDebug
UNEXPECTED TOP-LEVEL EXCEPTION: com.android.dx.util.DexEx
Received the following error
Execution failed for task ':app:transformDexArchiveWithDexMergerForDebug'.
com.android.build.api.transform.TransformException: com.android.dex.DexException: Multiple dex files define Landroid/support/constraint/ConstraintSet$1
Fix : go to Build -> Clean Project