"Error Loading Dependencies" when component is missing

[glennlawyer]

As per "Dash: A Pleasant and Productive Development Experience" https://github.com/plotly/dash/issues/125 I have found another cause of the "Error loading dependencies" error message.

The error fires when the component does not exist. Similar to already suspected problem:

@chriddyp

believe that the main cause is component property values that aren't the correct type.

I discovered the example when building a multi-page app. The dcc.Location object fires before the page with the component loads. I'll use the "hidden div" trick instead.

Minimal example. A cut and paste from https://dash.plot.ly/state with input 2 commented out and supress_callback_exceptions set to True.

# -*- coding: utf-8 -*-
import dash
import dash_core_components as dcc
import dash_html_components as html
from dash.dependencies import Input, Output, State

external_stylesheets = ['https://codepen.io/chriddyp/pen/bWLwgP.css']

app = dash.Dash(__name__, external_stylesheets=external_stylesheets)

app.config['suppress_callback_exceptions']=True 

app.layout = html.Div([
    dcc.Input(id='input-1-state', type='text', value='Montréal'),
    # dcc.Input(id='input-2-state', type='text', value='Canada'),
    html.Button(id='submit-button', n_clicks=0, children='Submit'),
    html.Div(id='output-state')
])


@app.callback(Output('output-state', 'children'),
              [Input('submit-button', 'n_clicks')],
              [State('input-1-state', 'value'),
               State('input-2-state', 'value')])
def update_output(n_clicks, input1, input2):
    return u'''
        The Button has been pressed {} times,
        Input 1 is "{}",
        and Input 2 is "{}"
    '''.format(n_clicks, input1, input2)


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