Steps to reproduce:

1. Open attached project
2. Build 'Test' scene for iOS
3. Run on a device with latest iOS 11 beta (15A5341f)
4. Click "Start Recording" and then choose "Record Screen" when the popup appears
5. Notice the recording fails with error:
'startRecordingWindowLayerContextIDs:windowSize:microphoneEnabled:cameraEnabled:broadcast:systemRecording:captureEnabled:listenerEndpoint:withHandler:', dropping incoming message and calling failure block.
Exception: accessing _cachedSystemAnimationFence requires the main thread

Notes:
Doesn't reproduce with earlier iOS versions (including earlier iOS 11 betas)
Broadcasting seems to work fine.