NSMutableArray : unrecognized selector sent to instance

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];
        }
    }
}

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 :

* Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[CA开发者_如何学GoLayerArray addObject:]: unrecognized selector sent to instance 0x4b36ce0'

I need your help ! Thanks !

You need to retain TGrid ! Otherwise, it will be deallocate by the autorelease pool and probably a CALayer takes its place in the memory !

Best option being to create a property with retain attribute out of it and access self.TGrid Don't forget to release it at the end (dealloc)

edit It is deallocated because every instance creator class method provides Autoreleased instance (that's a rule that everyone should follow and that Apple does follow in the whole SDK).