I migrated an Android app to AndroidX with the help of the Refactor option from Android Studio. After all the references in the java files were updated, I ran into the below error when I tried running my app.

  android.view.InflateException: Binary XML file line #2 in <>: Error inflating class androidx.constraintlayout.ConstraintLayout  
    Caused by: android.view.InflateException: Binary XML file line #2 in <>: Error inflating class androidx.constraintlayout.ConstraintLayout  
    Caused by: java.lang.ClassNotFoundException: androidx.constraintlayout.ConstraintLayout  

After a bit of head scratching and internet search, I was able to fix it by simply changing the declaration in the layout XML file. androidx.constraintlayout.ConstraintLayout to androidx.constraintlayout.widget.ConstraintLayout

I was expecting atleasst the IDE to throw an error, but had to find and fix it the hard way.