This answer is based on the assumption that you want the video to continue rendering while in the background.
I fixed this in my app doing by asking the OS to grant the app background task permisions (for a limited amount of time). This answer might help you too iOS generating a video in the background task
@property (nonatomic,assign) UIBackgroundTaskIdentifier __block backgroundRenderingID;
UIApplication *app = [UIApplication sharedApplication];
_backgroundRenderingID = [app beginBackgroundTaskWithExpirationHandler:^{
[app endBackgroundTask:_backgroundRenderingID];
_backgroundRenderingID = UIBackgroundTaskInvalid;
}];
Don't forget to let go once you are done!
[[UIApplication sharedApplication] endBackgroundTask:_backgroundRenderingID];