title: prepareForSegue getting called twice, with Attempt to present <UINavigationController> while presentation is in progress
body: <p>I am new to ios programming and asking here, but I visit all the time!  I am stumped at why I am getting this problem, it compiles with no errors and I have checked and checked all my outlets and identifiers in my MainStoryboard. </p>
  
  <p>I have 2 UITableViewControllers, I am passing a string from the first to the second when the user selects an item in the table, so in 
  FirstTableViewController.m</p>
  
  <pre><code>- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath
  {
      int sel = indexPath.row;
      if (sel == 0) {
          _keyName = [NSString stringWithString:_string1];
          NSLog(@"the table was selected at cell 0, %@", _string1);
      }
      if (sel == 1) {
          _keyName = [NSString stringWithString:_string2];
      }
      // more code below...
  }
  
  - (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender
  {
      if ([segue.identifier isEqualToString:@"ResultsSegue"])
      {
          UINavigationController *navigationController = segue.destinationViewController;
          ResultsViewController *rv = [[navigationController viewControllers] objectAtIndex:0];
          [rv setResults: _keyName];
          NSLog(@"in the progress view, %@", _keyName);
          //rv.delegate = (id)self;
          rv.delegate = self;            
      }
  }
  </code></pre>
  
  <p>And in my ResultsViewController.m</p>
  
  <pre><code>- (void)viewDidLoad
  {
      [super viewDidLoad];
      NSLog(@"in the results, %@", _results);
      NSLog(@"in the results view");
  }
  </code></pre>
  
  <p>In the NSlog readout I get:
  ...
  in the progress view, (null)
  in the results, (null)
  in the progress view, The Right String
  Warning: Attempt to present  on 
  
  <p>Then when I hit the cancel button to return to the firstTableview and press the detail view again it no longer shows null..</p>
  
  <p>in the progress view, The Right String
  in the results, The Right String
  in the progress view, The Right String</p>