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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 app.py

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

content = html.Div([
    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
‘intro_layout.py’, ‘main_layout.py’, ‘plots_layout.py’

What is wrong with this setup?

I filed a bug report to dbc components here: https://github.com/facultyai/dash-bootstrap-components/issues/476