500 Internal server error on jupyter notebook

import dash

import dash_core_components as dcc

import dash_html_components as html

app = dash.Dash()
app.Layout = html.Div('hi')

if __name__ == "__main__":

I get an error —> Internal Server Error

The server encountered an internal error and was unable to complete your request. Either the server is overloaded or there is an error in the application.

How to fix this?