I also had the same problem this morning.
I thought this was due to migrating to AndroidX but
It was wrong. It may occur due to improper indexing and building for e.g. the indexing is force stopped. or due to some IDE issues.
The isuue resolved by deleting gradle and .idea folder in project
and clearing cache of Android Studio IDE by D:\Users\ABCD.AndroidStudio3.2\system\caches.
And issue was resolved.
Note:- Please take Project Backups Daily.