Steps to reproduce:
1. Open the attached project
2. Open the available scene
3. Click the steps in the upper menu bar 'Asset Bundle Bug/Works/'
4. Notice that the scene playback works
5. Stop playback
6. Follow the steps in the upper menu bar 'Asset Bundle Bug/Does Not Work/'
7. Notice that the Unity Editor has frozen and become unresponsive

Note: This is caused by Building Asset Bundles only once
Note 2: Issue does not occur when Text Font is set to 'None' in Canvas

Expected: Editor does not freeze during playback

Reproduced with: 5.4.0b15, 5.3.4p4, (could not reproduce with earlier versions due to script incompatibility)

Update: fixed in 5.4.4p4