Issue : 
../../DevelopmentEnv/flutter/.pub-cache/hosted/pub.dartlang.org/tutorial_coach_mark-1.2.7/lib/tutorial_coach_mark.dart:95:55: Error: Method 'insert' cannot be called on 'OverlayState?' because it is potentially null.
 - 'OverlayState' is from 'package:flutter/src/widgets/overlay.dart' ('../../DevelopmentEnv/flutter/packages/flutter/lib/src/widgets/overlay.dart').
Try calling using ?. instead.
        Overlay.of(context, rootOverlay: rootOverlay).insert(_overlayEntry!);

Fix: added null check in line 95