Layout visible but getting id not found in layout

I am using this: Dash callback in a separate file - #16 by Philippe
to register callback in separate files and this: URL Routing and Multiple Apps | Dash for Python Documentation | Plotly for a multipage app.
The layout below is in my

fixed_content = html.Div([
        "Write intro here", id="intro"
        html.P("by Faith and Nicholas")

content = html.Div([
        id="fixed-content", style=CONTENT_STYLE
    html.Div(id="page-content", style=CONTENT_STYLE)

app.layout=html.Div([dcc.Location(id="url"), sidebar, content])

so content always has a fixed layout on top and depending on the URL, a switchable portion on the bottom.
It seems to work.

But I’m getting “id not found in layout” for so many elements:

I have 3 other files for page-content
‘’, ‘’, ‘’

What is wrong with this setup?

I filed a bug report to dbc components here: