iOS开发，调用相机iOS7 出现黑屏，iOS8以上正常， 以下为代码，求大神指教

//调用相机
if (IOS8) {
            
            //先设定sourceType为相机，然后判断相机是否可用（ipod）没相机，不可用将sourceType设定为相片库
            UIImagePickerControllerSourceType sourceType = UIImagePickerControllerSourceTypeCamera;
            if (![UIImagePickerController isSourceTypeAvailable: UIImagePickerControllerSourceTypeCamera]) {
                sourceType = UIImagePickerControllerSourceTypePhotoLibrary;
            }
            
            
            UIImagePickerController *picker = [[UIImagePickerController alloc] init];//初始化
            picker.delegate = self;
            
            picker.allowsEditing = YES;//设置可编辑
            picker.sourceType = sourceType;
            [self presentViewController:picker animated:YES completion:^{
                
                
            }];//进入照相界面
            
        }else {
            UIImagePickerControllerSourceType sourceType = UIImagePickerControllerSourceTypeCamera;
            if (![UIImagePickerController isSourceTypeAvailable: UIImagePickerControllerSourceTypeCamera]) {
                sourceType = UIImagePickerControllerSourceTypePhotoLibrary;
            }
            
            UIImagePickerController *picker = [[UIImagePickerController alloc] init];
            picker.delegate = self;
            picker.allowsEditing = YES;
            picker.sourceType = sourceType;
            [self presentModalViewController:picker animated:YES];
            
            
        }