title: How to create nullable variable in Dart
body: <p>I created a model class, this is one of my a variable in model class</p>
  
  <pre><code>Datetime hotel_effect_open_date
  </code></pre>
  
  <p>but JSON response hotel_effect_open_date is null, getting an error in my application. and I modified to DateTime to String, it's working. In API created in the .net core, it looks like this,</p>
  
  <pre><code>Nullable&lt;DateTime&gt; hotel_effect_open_date
  </code></pre>
  
  <p>How to create a nullable variable in DART language?</p>