Reproduction steps:

1. Open the attached project.
2. Build and run it for iOS.
3. Click the "Set Flag" button.
4. Check the console:

UnityException: SetNoBackupFlag can only be called from the main thread.
Constructors and field initializers will be executed from the loading thread when loading a scene.
Don't use this function in the constructor or field initializers, instead move initialization code to the Awake or Start function.

Reproduced with: 5.3.7p1, 5.6.0b1