Hi, I believe I've found some unexpected behavior with the new dev-tools UI feature.

When a dropdown's options contains a null/NaN value - an error appears in the new UI and the dropdown is not rendered in the browser (however no errors are shown in the console). If the app is run with debug=False, the dropdown renders and the nan value appears in the options (but cannot be selected?).

Unfortunately I'm not sure what the expected behavior should be in this scenario - or if this is a valid issue. Please feel free to remove if not.

import dash
import dash_core_components as dcc
import dash_html_components as html
import numpy as np
import pandas as pd

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

dropdown_list = pd.Series([1,2,3,np.nan])

app.layout = html.Div(children=[
    dcc.Dropdown(
        id='dropdown',
        options=[{'label':f"Option: {i}", 'value':i} for i in dropdown_list]
    )
])

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