title: No rebuild after choosing value in Dropdown
body: <p>I am sending the choosen value of the DropdownMenu via Callback to the parent widget (see code below).</p>
  <pre><code>    return DropdownButton&lt;String&gt;(
        hint: Text(widget.hint),
        value: valueChoose,
        items: widget.users.map((dynamic valueItem) {
          return DropdownMenuItem&lt;String&gt;(
            value: valueItem,
            child: Text(valueItem),
          );
        }).toList(),
        onChanged: (newValue) {
          setState(() {
            valueChoose = newValue;
            besetzungsList.add(valueChoose);
            widget.besetzungsListChanged(valueChoose);
            widget.fromDropDown(true);
          });
        },
      );
  </code></pre>
  <p>The problem is with &quot;besetzungsList[i] = value;&quot; no rebuild occurs. But I need the choosen value to be shown in the UI. If I set the value via the insert function then it works. But I need to replace the value inside the list.</p>
  <pre><code>  DataCell(DropDown(
                    hint: &quot;Wählen&quot;,
                    users: users,
                    besetzungsListChanged: (String value) {
                      besetzungsList[i] = value;
                    },
                    fromDropDown: (bool value) =&gt; fromDropDown = value,
                  ))
  
  </code></pre>