全部清除时的Dash dcc.Dropdown问题

import dash
import dash_core_components as dcc
import dash_html_components as html

app = dash.Dash(__name__)

options = [
    {'label': 'Option 1', 'value': 'option1'},
    {'label': 'Option 2', 'value': 'option2'},
    {'label': 'Option 3', 'value': 'option3'}
]

app.layout = html.Div([
    dcc.Dropdown(
        options=options,
        value=None,
        clearable=True
    ),
    html.Div(id='output')
])

@app.callback(
    dash.dependencies.Output('output', 'children'),
    [dash.dependencies.Input('dropdown', 'value')]
)
def update_output(value):
    if value is None:
        return 'No option selected.'
    else:
        return f'Selected option: {value}'

if __name__ == '__main__':
    app.run_server(debug=True)