title: AVCaptureSession fails when returning from background
body: <p>I have a camera preview window which is working well 90% of the time.  Sometimes however, when returning to my app if it's been in the background, the preview will not display.  This is the code I call when the view loads:</p>
  
  <pre><code>- (void) startCamera {
  
  session = [[AVCaptureSession alloc] init];
  session.sessionPreset = AVCaptureSessionPresetPhoto;
  
  AVCaptureVideoPreviewLayer *captureVideoPreviewLayer = [[AVCaptureVideoPreviewLayer alloc] initWithSession:session];
  captureVideoPreviewLayer.frame = _cameraView.bounds;
  [_cameraView.layer addSublayer:captureVideoPreviewLayer];
  captureVideoPreviewLayer.videoGravity = AVLayerVideoGravityResizeAspectFill;
  captureVideoPreviewLayer.position=CGPointMake(CGRectGetMidX(_cameraView.bounds), 160);
  
  AVCaptureDevice *device = [AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeVideo];
  NSError *error = nil;
  
  AVCaptureDeviceInput *input = [AVCaptureDeviceInput deviceInputWithDevice:device error:&amp;error];
  if (!input) {
  
      NSLog(@"ERROR: %@", error);
  
  
      UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"Important!"
                                                      message:@"Unable to find a camera."
                                                     delegate:nil
                                            cancelButtonTitle:@"Ok"
                                            otherButtonTitles:nil];
      [alert show];
      [alert autorelease];
  }
  
  [session addInput:input];
  
  stillImage = [[AVCaptureStillImageOutput alloc] init];
  NSDictionary *outputSettings = [[NSDictionary alloc] initWithObjectsAndKeys:AVVideoCodecJPEG , AVVideoCodecKey, nil];
  [stillImage setOutputSettings:outputSettings];
  
  [session addOutput:stillImage];
  [session startRunning];
  }
  </code></pre>
  
  <p>If this happens, I can switch to my preferences view and back again and al is well,but it's an annoying bug I'd like to kill.  The preview window is a UIView in my storyboard.</p>