NSMutableArray : 无法识别的选择器发送到实例

我正在尝试使用 NSMutableArray 的 NSMutableArray 存储数组 int[9][9]，其中我存储了数组中的 81 个整数:

I'm trying to store an array int[9][9] with a NSMutableArray of NSMutableArray where I store my 81 integers from the array :

- (void)awakeFromNib {
    // initialization matrix
    for (int i = 0; i < 9; i++) {
        for (int j = 0; j < 9; j++) {
            matrix[i][j] = 0;
        }
    }

        // Creating NSMutableArray instance
    TGrid = [NSMutableArray arrayWithCapacity:10];

    [self saveGrid];
}

- (void)saveGrid {
    NSNumber *aInt;
    NSMutableArray *Grid = [NSMutableArray arrayWithCapacity:81];
    for (int i = 0; i < 9; i++) {
        for (int j = 0; j < 9; j++) {
            aInt = [NSNumber numberWithInt:matrix[i][j]];
            [Grid addObject:aInt];
        }
    }
    [TGrid addObject:Grid];
}

- (IBAction)undo:(id)sender {
    [TGrid removeLastObject];
    NSMutableArray *Grid = [TGrid lastObject];
    for (int i = 0; i < 9; i++) {
        for (int j = 0; j < 9; j++) {
            matrix[8-i][8-j] = [[Grid lastObject] intValue];
            [Grid removeLastObject];
        }
    }
}

当 saveGrid 第一次被awakeFromNib 方法调用时，它起作用了.但是当我更改矩阵时，它再次调用 saveGrid，这次我收到此错误:

When saveGrid is first called by the awakeFromNib method, it works. But when I change my matrix, it calls again saveGrid, and this time I get this error :

* 由于未捕获的异常而终止应用'NSInvalidArgumentException'，原因:'-[CALayerArray addObject:]:无法识别的选择器发送到实例0x4b36ce0'

* Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[CALayerArray addObject:]: unrecognized selector sent to instance 0x4b36ce0'

我需要你的帮助！谢谢！

I need your help ! Thanks !