When Animation Track is added in 'Track Sub-Group' and Preset is created for 'Track Group' then on deleting that 'GroupTrack' Preset or simply selecting Cube Gameobject in Hierarchy resulting editor throws following errors.
1] NullReferenceException: Object reference not set to an instance of an object
2] GUI Error: You are pushing more GUIClips than you are popping.
Stack trace and video attached for reference.

Steps to repro:
1. Create a new Project
2. Windows > Timeline.
3. Select Cube and create Cube Timeline.
4. Track Group >() > Track Sub-Group > () >Animation Track.
5. Select Track Group and make a Preset > 'GroupTrack' Preset > Timeline >Select Animation Track.
or
Select Track Group and make a Preset > 'GroupTrack' Preset > Delete
6. Observe the Errors in the console window.

Actual Result:
Errors are thrown on deleting 'GroupTrack' Preset when Animation Track is added in Track Sub-Group.

Expected Result:
After deleting GroupTrack Preset, Animation Track accessible without any error in console.

Reproducible on:
2020.2.0b5, 2020.2.0a16

Working fine on:
2020.2.0a15, 2020.1.7f1 , 2019.4.11f1

Environment:
Occurring on Windows 10 & macOS 10.15.