What is meant by all components within your callback must exist in the layout when your app starts

Dash Python
1

In Callback Gotchas I’ve read that:

For full validation, all components within your callback must exist in the layout when your app starts, and you will see an error if they do not.

Below is a piece of code where, as far as I see, not all components exist in the layout when the application starts. This is because 3 components (dcc.Input, html.Button and html.Div) are only set when the callback entry returns. However, this code seems to work just fine.

So how much should I be concerned about this

Callbacks require their Inputs , States , and Output to be present in the layout

section in Callback Gotchas?

from dash import Dash, Input, Output, State, dcc, html
from dash.dash import PreventUpdate

external_stylesheets = ['https://codepen.io/chriddyp/pen/bWLwgP.css']
app = Dash(__name__, external_stylesheets=external_stylesheets)

app.layout = html.Div(
    id='main',
    children=[
        html.Button(
            id='load',
            children='Load'

        ),
    ]
)


@app.callback(
    Output('main', 'children'),
    Input('load', 'n_clicks')
)
def entry(n_clicks):
    if n_clicks is None:
        raise PreventUpdate

    return html.Div([
        dcc.Input(id='input', value='initial value'),
        html.Button(id='button', children='Submit'),
        html.Div(id='output')
    ]
    )


@app.callback(
    Output('output', 'children'),
    Input('button', 'n_clicks'),
    State('input', 'value')
)
def echo(n_clicks, value):
    if n_clicks is None:
        raise PreventUpdate

    return value


if __name__ == '__main__':
    app.run_server()
2

FYI