XML Parser blocking main thread and dispatch_async crashing app - ios

Question

i am developing an application and in this app i am loading xml data in a uitableview and that table view call parse method after every 5 secs and reload uitable to load new data. Everything was working fine but app got stuck when parsing starts after 5 sec so i decided to implement dispatch_async in parse method but after that application is crashing like after 5 sec whenever app reload uitable. here is my code.
- (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];
}
}
});
});
}
Method through which i am sending a message to the xml data file.
- (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 = #"";
}
And the Error which i am getting is:

Answer 1

Print your array. Check it twice. I think you are getting an empty array. Print every object in the console which you get from server and which you parse. So you will get idea.
Update:
Just reload the table data on the main thread and don't parse data on it. like:
dispatch_async(dispatch_get_main_queue(), ^{
[messageList reloadData];
});
Put the other code outside the main thread.