problem with ComponentRegistry and late imports

Late imports of components libraries are not served on the index if the app has already started.

Example

import dash
import dash_html_components as html

from dash.exceptions import PreventUpdate
from dash.dependencies import Output, Input

app = dash.Dash(__name__)
app.layout = html.Div([
    html.Button('click', id='click'),
    html.Div(id='output')
])


@app.callback(Output('output', 'children'), [Input('click', 'n_clicks')])
def on_click(n_clicks):
    if n_clicks is None:
        raise PreventUpdate

    import dash_core_components as dcc

    return dcc.Input(id='input', value='my-value')


if __name__ == '__main__':
    app.run_server(debug=True, port=8051)

The button will not be inserted and the console show error:

Error: dash_core_components was not found.

https://github.com/T4rk1n/pytest-dash/issues/15

Start at ComponentRegistry and the index-serving path, then reproduce the provided callback example with a late import of dash_core_components. Done means the dynamically returned component is served after the app starts and the button inserts it without the “dash_core_components was not found” error.