title: DateTimeFormField is not updating after setState call (need more than just giving initialValue? I don't know how to use a controller here)
body: <p>I have a problem of refreshing my DateTimeFormField after a setState call, do someone having the same problem or I am just missing the right code?</p>
  <p>I am using:</p>
  <pre><code>date_field: ^3.0.0
  </code></pre>
  <p>Here is my code:</p>
  <pre><code>DateTimeFormField(
      initialValue: myInitialValue,
      decoration: const InputDecoration(
          suffixIcon: Icon(Icons.event_note),
          labelText: 'Time line',
       ),
       mode: DateTimeFieldPickerMode.date,
       onDateSelected: (DateTime value) {
         myInitialValue = value;
       },
  ),
  </code></pre>
  <p>Then hope to see the date changing after a:</p>
  <pre><code>setState(() { myInitialValue = value; });
  </code></pre>
  <p>I was hoping to have something like a controller here but can't find it.</p>
  <p>Thanks in advance</p>