title: [__NSCFString appendString:]: nil argument' error
body: <p>I was trying to show string values in print preview option.If i didn't enter any text in any of the textfield and while clicking print button the app gets crashed.And the error is [__NSCFString appendString:]: nil argument'.Here is my code</p>
  
  <pre><code>-(NSMutableString*)pageStringToPrint {
  
      NSMutableString * cnoteString = [[NSMutableString alloc]init];
  
      [cnoteString setString:@"Class: "];
      [cnoteString appendString:self.cnote.classname];
      [cnoteString appendString:@"\nTeacher: "];
      [cnoteString appendString:self.cnote.teacher];
      [cnoteString appendString:@"\nSource: "];
      [cnoteString appendString:self.cnote.source];
      [cnoteString appendString:@"\nTopic: "];
      [cnoteString appendString:self.cnote.topic];
  
      NSDateFormatter *dateFormat = [[NSDateFormatter alloc] init];
      [dateFormat setDateFormat:@"MM-dd-yyyy"];
      NSString *dateString = [dateFormat stringFromDate:self.cnote.date];
      [cnoteString appendString:@"\nDate: "];
      [cnoteString appendString: dateString];  
  
      cnoteString = [NSMutableString stringWithFormat:@"&lt;div id=\"mainContainer\"&gt;&lt;div id=\"header\"&gt;%@&lt;/div&gt;",[cnoteString kv_encodeHTMLCharacterEntities]];
  </code></pre>
  
  <p>This is how i created Self.cnoe.classname</p>
  
  <pre><code>- (void)viewDidLoad
  {
      [super viewDidLoad];
      // Do any additional setup after loading the view.
      [self saveCNote ];
      self.classNameTextField.text = self.cnote.classname;
      self.topicTextField.text = self.cnote.topic;
      self.sourceTextField.text = self.cnote.source;
      self.teacherTextField.text = self.cnote.teacher;
  
  
      self.essentialQuestionTextView.placeholder = @"Essential Question";
      self.essentialQuestionTextView.layer.cornerRadius = 7.0f;
      self.essentialQuestionTextView .layer.borderColor = [[UIColor grayColor] CGColor];
      self.essentialQuestionTextView.layer.borderWidth= 1.0f;
      self.essentialQuestionTextView.delegate = self;
      self.essentialQuestionTextView.text = self.cnote.essentialQuestion;
  
      // set the date
      self.dateTextField.delegate = self;
      NSDateFormatter *dateFormat = [[NSDateFormatter alloc] init];
      [dateFormat setDateFormat:@"MM-dd-yyyy"];
      self.dateTextField.text = [dateFormat    stringFromDate:self.cnote.date];
    }
  </code></pre>
  
  <p>Save Cnote:</p>
  
  <pre><code>- (void)saveCNote {
  
      self.cnote.classname = self.classNameTextField.text;
      self.cnote.topic = self.topicTextField.text;
      self.cnote.source = self.sourceTextField.text;
      self.cnote.teacher = self.teacherTextField.text;
  
  
      [self.cnote save];
  }
  </code></pre>