title: Python dash dropdown placeholder text
body: <p>I am trying to build a dropdown menu in dash, where the text of the placeholder should be defined by another callback.</p>
  <p>What I tried so far is to have the placeholder defined by a html.Div, like this: <code>dcc.Dropdown(id = 'test_dropdown', options = [{'label': 'Test1', 'value': '1'},{'label': 'Test2', 'value': '2'}], placeholder = html.Div(id = 'test_placeholder'))</code>, with the text being defined by the other callback. This resulted in the following error message: <em>Invalid argument <code>placeholder</code> passed into Dropdown with ID &quot;test_dropdown&quot;.
  Expected <code>string</code>.
  Was supplied type <code>array</code>.</em></p>
  <p>If I remove the square brackets around the html.Div, the error message becomes: <em>The <code>dash_core_components.Dropdown</code> component (version 1.16.0) with the ID &quot;test_dropdown&quot; detected a Component for a prop other than <code>children</code>
  Did you forget to wrap multiple <code>children</code> in an array?
  Prop placeholder has value Div(id='test_placeholder')</em></p>
  <p>Has anyone got suggestions on how I could implement a placeholder that depends on another dropdown? Or would it be better to implement multiple dropdowns, and hide all but one using a hidden html.Div?</p>
  <p>Thanks in advance for your help</p>