title: Exception thrown if string is null
body: <p>I am getting an exception if a string is null:</p>
  
  <pre><code>Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[NSNull length]: unrecognized selector sent to instance
  </code></pre>
  
  <p>This is one of the objects of the dictionary,that contains a null string:</p>
  
  <pre><code>{
      cabdriver = "&lt;null&gt;";
      code = SV1000000079;
      date = "2015-03-15";
      destiny = "";
      email = "jose@gmail.com";
      origin = vxd;
  }
  </code></pre>
  
  <p>And this is my code where the exception is thrown:</p>
  
  <pre><code>- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
  {
      HistorialCellTableViewCell *cell = (HistorialCellTableViewCell *)[tableView dequeueReusableCellWithIdentifier:@"Cell"];
  
      NSString *origin = [[historialServicios objectAtIndex:indexPath.row] valueForKey:@"origin"];
      if (origin.length ==0){
          cell.origen_label.text = @"-";
      } else {
          cell.origen_label.text = origin;
      }
  
      NSString *destiny = [[historialServicios objectAtIndex:indexPath.row] valueForKey:@"destiny"];
      if (destiny.length ==0){
          cell.destiny_label.text = @"-";
      } else { 
          cell.destiny_label.text = destiny;
      }
  
  
      NSString *cabdriver = [[historialServicios objectAtIndex:indexPath.row] valueForKey:@"cabdriver"];
      if (cabdriver.length ==0){
          cell.conductor_label.text = @"-";
      } else {
          cell.conductor_label.text = cabdriver;
      }
  
      NSString *date = [[historialServicios objectAtIndex:indexPath.row] valueForKey:@"date"];
  
      cell.fecha_label.text = date;
      return cell;
  }
  </code></pre>
  
  <p>I have tried other ways to check if the string is null, but I have been searching and this should be the way to check it. </p>
  
  <p>Please tell me what is wrong in my code?</p>