title: Exception when trying to convert TextBox content
body: <p>It is possible to write the following condition? </p>
  
  <pre><code>if (textbox.text != null)
  </code></pre>
  
  <p>Because when I have  written this condition in my program, <strong>it throws an exception.</strong></p>
  
  <p>If the data type of <code>textbox</code> is <code>string</code> then it is also nullable ?</p>