问NSMutableDictionary“无法识别的选择器已发送到实例”

EN

    -(void)saveDictionary:(int)counter
    {
        NSString *path=[[NSBundle mainBundle] pathForResource:@"Data" ofType:@"plist"];
        NSString *test = [NSString stringWithFormat:@"%d", counter];
        [theDictionary setObject:test forKey:@"Counter"]; <---- Error
        [theDictionary writeToFile:path atomically:YES];
    }

    - (void)applicationDidEnterBackground:(UIApplication *)application
    {
        [self saveDictionary:[_viewController counter]];
    }

-[NSCFString setObject:forKey:]: unrecognized selector sent to instance
*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[NSCFString setObject:forKey:]: unrecognized selector sent to instance

-(void)initDictionary {
    if (theDictionary == nil) {
        NSString *path=[[NSBundle mainBundle] pathForResource:@"Data" ofType:@"plist"];
        theDictionary = [[NSMutableDictionary alloc] initWithContentsOfFile:path];
        theString = [theDictionary objectForKey:@"Counter"];
    }
}

theString = [[NSString alloc] initWithFormat:[theDictionary objectForKey:@"Counter"]];