Why is this dash app perpetually "Updating..."

Dash Python
1

I have a dash app which uses a dcc.Location type pattern to populate a page content div. But if I add a callback that populates an element of the page content with an Input that does not exist, the page never finished loading, and is stuck in ‘Updating…’. Here’s a minimal working example:

import dash
import dash_html_components as html
from dash.dependencies import Input, Output

app = dash.Dash(__name__)
server = app.server
app.config["suppress_callback_exceptions"] = True

app.layout = html.Div(
    [dcc.Location(id="url", refresh=False), html.Div(id="layout-div")]
)


@app.callback(
    Output(component_id="layout-div", component_property="children"),
    [Input(component_id="url", component_property="pathname")],
)
def update_layout_div(url):
    return [html.Div(id="div-test")]


@app.callback(
    Output(component_id="div-test", component_property="children"),
    [Input(component_id="nonexistent-input", component_property="value")],
)
def example_callback(input):
    return "never triggered"


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

Is it obvious why this doesn’t work? I feel that I’ve used this pattern before. I thought that the callback just shouldn’t be called until the nonexistent-input input exists.