UIDocumentInteractionController在退出时崩溃

-(id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil
{
    self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];

    documentPath = [[NSBundle mainBundle] pathForResource:@\"file\" ofType:@\"pdf\"];
    NSURL *targetURL = [NSURL fileURLWithPath:documentPath];

    document = [UIDocumentInteractionController interactionControllerWithURL: targetURL];
    document.delegate = self;
    [document retain];

    return self;
}

-(UIViewController *)documentInteractionControllerViewControllerForPreview: (UIDocumentInteractionController *) controller
{
    return self;
}

-(void)documentInteractionControllerDidEndPreview:(UIDocumentInteractionController *)controller
{
    [document autorelease];
}

-(void)viewDidLoad
{
    [super viewDidLoad];

    [document presentPreviewAnimated: YES]; // ** CRASH **
}

-(void)viewDidUnload
{
    [super viewDidUnload];
}

-(void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];
}

-(BOOL)shouldAutorotateToInterfaceOrientation:(UIInterfaceOrientation)interfaceOrientation
{
    return YES;
}

-(void)dealloc
{
    [super dealloc];
}