title: Error trying to set text field text to string
body: <p>I am trying to set my text field's text equal to a saved string. I have it global because I need to access it at another point. I get this error: <code>
  Undefined symbols for architecture arm64:
    "_tailNum", referenced from:
        -[rvc newPlaneClicked:] in rvc.o
    "_MM", referenced from:
        -[rvc newPlaneClicked:] in rvc.o
  ld: symbol(s) not found for architecture arm64
  clang: error: linker command failed with exit code 1 (use -v to see invocation)</code></p>
  
  <p>What is causing this/how is it fixed?</p>
  
  <pre><code>extern NSString *tailNum;
  extern NSString *MM;
  
  - (void)newPlaneClicked:(id)sender {
  
      UIAlertView *newPlaneAlert = [[UIAlertView alloc]initWithTitle:@"FlightLog Pro" message:@"Enter tail number and make/model:"delegate:self cancelButtonTitle:@"Cancel" otherButtonTitles:@"Save", nil];
  
      newPlaneAlert.delegate = self;
      [newPlaneAlert setAlertViewStyle:UIAlertViewStyleLoginAndPasswordInput];
  
      UITextField *enterNewPlaneTail = [newPlaneAlert textFieldAtIndex:0];
      UITextField *enterNewPlaneMM = [newPlaneAlert textFieldAtIndex:1];
  
      [enterNewPlaneTail setPlaceholder:@"Click to enter a make/model."];
      [enterNewPlaneMM setPlaceholder:@"Click to enter a plane tail number."];
  
      [newPlaneAlert textFieldAtIndex:0].autocapitalizationType = UITextAutocapitalizationTypeSentences;
      [newPlaneAlert textFieldAtIndex:0].autocorrectionType = YES;
  
      [newPlaneAlert textFieldAtIndex:1].autocapitalizationType = UITextAutocapitalizationTypeSentences;
      [newPlaneAlert textFieldAtIndex:1].autocorrectionType = YES;
      [newPlaneAlert textFieldAtIndex:1].secureTextEntry = NO;
  
      [newPlaneAlert show];
  
      enterNewPlaneTail.text = tailNum;
      enterNewPlaneMM.text = MM;
  }
  </code></pre>