AVCaptureSession error when returning from background

I have a camera preview window that works well 90% of the time. Sometimes, however, when you return to my application, if it was in the background, the preview will not be displayed. This is the code that I call when the view is loaded:

- (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:&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];
}

If this happens, I can switch to my view mode and vice versa, and everything is fine, but this is an annoying mistake that I would like to kill. The preview window is a UIView in my storyboard.