XML Parser阻止主线程和dispatch_async崩溃的应用程序

我正在开发一个应用程序，在此应用程序中，我将XML数据加载到uitableview中，并且该表视图每隔5秒就会调用解析方法并重新加载uitable以加载新数据。一切工作正常，但5秒钟后开始解析时应用卡住了，所以我决定在解析方法中实现dispatch_async，但是此应用崩溃后，只要5秒钟后就可以重新加载应用了。这是我的代码。

- (void) Parse{
    previusCount = rssOutputData.count;
    rssOutputData = [[NSMutableArray alloc]init];

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
     NSString *post =[[NSString alloc] initWithFormat:@"https://messages_%@.xml",[[NSUserDefaults standardUserDefaults] stringForKey:@"xmls_id"]];
     NSData *xmlData=[[NSData alloc]initWithContentsOfURL:[NSURL URLWithString:post]];
     xmlParserObject =[[NSXMLParser alloc]initWithData:xmlData];
     [xmlParserObject setDelegate:self];
        dispatch_async(dispatch_get_main_queue(), ^{
     [xmlParserObject parse];
         [messageList reloadData];
         if (previusCount != rssOutputData.count) {
             NSInteger bottomRow = [rssOutputData count] - 1; // this is your count's array.
             if (bottomRow >= 0) {
      ///////getting to latest msg/////////////
                 NSIndexPath *indexPathnew = [NSIndexPath indexPathForRow:bottomRow inSection:0];
                 [self.messageList scrollToRowAtIndexPath:indexPathnew atScrollPosition:UITableViewScrollPositionTop animated:YES];
             }
         }
     });
    });
}

我将消息发送到xml数据文件的方法。

- (IBAction)sendClicked:(id)sender {
    [messageText resignFirstResponder];
    if ( [messageText.text length] > 0 ) {
        NSString *rawStr;
    if ([[NSUserDefaults standardUserDefaults] integerForKey:@"userType"] == 1) { 
        rawStr = [NSString stringWithFormat:@"data=%@&user_id=%@&session_id=%@", messageText.text, [[NSUserDefaults standardUserDefaults] stringForKey:@"therapist_id"],[[NSUserDefaults standardUserDefaults] stringForKey:@"xmls_id"]];
    } else{//////In case of Patient
        rawStr = [NSString stringWithFormat:@"data=%@&user_id=%@&session_id=%@", messageText.text, [[NSUserDefaults standardUserDefaults] stringForKey:@"patient_id"],[[NSUserDefaults standardUserDefaults] stringForKey:@"xmls_id"]];
    }
        NSData *data = [rawStr dataUsingEncoding:NSUTF8StringEncoding];

        NSURL *url = [NSURL URLWithString:@"http://do_add_message.php"];
        NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:url];

        [request setHTTPMethod:@"POST"];
        [request setHTTPBody:data];
        NSURLResponse *response;
        NSError *err;
        NSData *responseData = [NSURLConnection sendSynchronousRequest:request returningResponse:&response error:&err];
        NSLog(@"responseData: %@", responseData);
        //////////////////////
        [self Parse];
    }

    messageText.text = @"";
}

我得到的错误是：