iphone MPMoviePlayerViewController CGContext错误

我正在为iPhone制作一个应用程序,该应用程序将使用MPMoviePlayerViewController播放一些电影.到目前为止,我已经得到了播放电影,但是它在调试器中抛出一些错误,所有这些都从CGContext开始.我试图修复我的大脑.以下是我的代码的细节：

.h文件：

#import <UIKit/UIKit.h>
#import <MediaPlayer/MediaPlayer.h>
@interface MovieViewController : UIViewController {
    MPMoviePlayerViewController *playerController;
}

-(IBAction) playMovie:(id)sender;

.m文件：

@interface MovieViewController ()
@end
@implementation

-(IBAction)playMovie:(id)sender {
NSURL *url = [NSURL fileURLWithPath:[[NSBundle mainBundle]
                                     pathForResource:@"moviename" ofType:@"mp4"]];
playerController =  [[MPMoviePlayerViewController alloc]
                     initWithContentURL:url];
[self presentMoviePlayerViewControllerAnimated:playerController];
playerController.moviePlayer.movieSourceType = MPMovieSourceTypeFile;
[playerController.moviePlayer play];
playerController = nil; 
}

当我运行程序时,电影将播放,但是当代码执行时：playerController = [[MPMoviePlayerViewController alloc] initWithContentURL：url];发生以下错误：

<Error>: CGContextSaveGState: invalid context 0x0
<Error>: CGContextClipToRect: invalid context 0x0
<Error>: CGContextTranslateCTM: invalid context 0x0
<Error>: CGContextDrawShading: invalid context 0x0
<Error>: CGContextRestoreGState: invalid context 0x0

我知道无效的上下文0x0意味着这些特定的变量没有值,但我不知道如何补救这个.任何帮助将不胜感激.

MPMoviePlayerViewController* mpvc = 
    [[MPMoviePlayerViewController alloc] initWithContentURL: m];

UIGraphicsBeginImageContext(CGSizeMake(1,1));
MPMoviePlayerViewController* mpvc = 
    [[MPMoviePlayerViewController alloc] initWithContentURL: m];
UIGraphicsEndImageContext();