发送到实例的无法识别的选择器“问题

我的代码在途中某处中断，并在使用导航栏按钮时崩溃。

my code broke somewhere along the way, and crashes when using the navigation bar buttons.

错误消息：
** *由于未捕获的异常'NSInvalidArgumentException'而终止应用程序，原因：'-[[UIView newMemoViewController：didAddMemo：]：无法识别的选择器发送到实例0x5b55a60'

在调试时，程序确实运行 cancel 方法，并在 @synthesize 行引发异常。但是，我看不出有什么问题。

When debugging, the program does run the cancel method, and throws an exception at the @synthesize line. However, I cannot see anything wrong with it.

症状是相同的，因此我只包含有关取消按钮的相关代码：

The symptoms are identical, so I am including the relevant code only for the Cancel button:

NewMemoViewController.h

#import <UIKit/UIKit.h>
@protocol NewMemoDelegate;


@class AKVoiceMemo;


@interface NewMemoViewController : UIViewController {
    @private
        AKVoiceMemo *voiceMemo;
        id <NewMemoDelegate> delegate;
}

@property (nonatomic, retain) AKVoiceMemo *voiceMemo;
@property (nonatomic, assign) id <NewMemoDelegate> delegate;

@end

@protocol NewMemoDelegate <NSObject>
- (void)newMemoViewController:(NewMemoViewController *)newMemoViewController didAddMemo:(AKVoiceMemo *)voiceMemo;


@end

NewMemoViewController.m

#import "NewMemoViewController.h"

@synthesize delegate;


- (void)viewDidLoad {
    UIBarButtonItem *cancelButtonItem = [[UIBarButtonItem alloc] initWithTitle:@"Cancel" style:UIBarButtonItemStyleBordered target:self action:@selector(cancel)];
    self.navigationItem.leftBarButtonItem = cancelButtonItem;
    [cancelButtonItem release];
}


- (void)cancel {
    [self.delegate newMemoViewController:self didAddMemo:nil];
}

我们将不胜感激。

编辑：委托是 RootViewController ：

- (void)newMemoViewController:(NewMemoViewController *)newMemoViewController didAddMemo:(AKVoiceMemo *)voiceMemo {
    if (voiceMemo){
        // Show the note in a new view controller
        // TODO: Implement this
    }

    [self dismissModalViewControllerAnimated:YES];
}