text: Title: datepicker doesn't open if date values are `None`
  Body: Originally reported in https://community.plot.ly/t/datepickerrange-clearable-in-tabs-not-working-as-expected/6850/4
  
  ```python
  import dash
  from dash.dependencies import Input, Output
  import dash_core_components as dcc
  import dash_html_components as html
  from datetime import datetime as dtime
  
  app = dash.Dash()
  
  
  app.config['suppress_callback_exceptions'] = True
  app.layout = html.Div([
      dcc.DatePickerRange(
          id='Dr1',
          clearable=True,
          reopen_calendar_on_clear=True,
          start_date_placeholder_text='Select a date',
          end_date=dtime.today().strftime("%Y-%m-%d")
      ),
      html.Div(id='output')
  ])
  
  
  @app.callback(
      Output('output', 'children'),
      [Input('Dr1', 'start_date'),
       Input('Dr1', 'end_date')])
  def dates_selected(start_date, end_date):
      value = "From- %s  To-   %s" % (start_date, end_date)
      return value
  
  
  if __name__ == '__main__':
      app.run_server(debug=True)
  ```
  
  Quite a few error messages in the console
  ![image](https://user-images.githubusercontent.com/1280389/33187985-ec8ca686-d063-11e7-9ce2-e4cfae76a676.png)