title: dash-plotly progress bar in browser does not show up
body: <p>I have something like this in my layout. </p>
  
  <pre><code>import dash
  import dash_html_components as html
  import dash_core_components as dcc
  import dash_bootstrap_components as dbc
  
  
  app = dash.Dash()
  
  app.layout = html.Div([
  
      html.H1('Progress bar'),
      dbc.Progress(id="progress", value=50, striped=True, animated=True)
  ])
  
  app.run_server(debug=True, port=9995)
  </code></pre>
  
  <p>I expect a progress bar at a value of 50% showing up in my dash app. But nothing shows up. Any idea why?</p>