Repro steps:
1. Open DoorA Animator Controller in the Animator window.
2. Start clicking on the states/connections (i.e. Idle to opening transition to Open).
3. Unity hangs

Expected outcome: to be able to select transition arrows.

Notes:
- Assets are from Tiny Dungeons package by VespaWarrior.
- Project originally opened in 5.3.1f1 (works fine) and then upgaded to 5.4.0b7, spin 3 (hangs here).
- Reopening the project in 5.3.1f1 fixes the issue.
- Discovered by playing the test level. Door plays opening sound and opening animation in 5.3.1f1, but plays only the sound in 5.4.0b7
- Regression; worked in 5.3.1f1 up to 5.4.0b6