[BUG] Multi-page Dash app breaks with `suppress_callback_exceptions=True` in latest version

[petar-qb]

Describe your context

dash                 3.1.0

Describe the bug

In dash==3.1.0, multi-page app fails when suppress_callback_exceptions=True. This issue does not occur in dash==3.0.4 or when the flag is unset (because it defaults to False).

Here's the minimal reproducible example:

import dash
from dash import Dash, html

print(dash.__version__)  # 3.1.0

app = Dash(
    use_pages=True,
    pages_folder="",
    suppress_callback_exceptions=True,
)
dash.register_page("/", layout=lambda: html.H1("Homepage"))

app.layout = html.Div([
    dash.page_container,
])

app.run(debug=True)

Error message:

  File "/lib/python3.12/site-packages/dash/dash.py", line 2497, in router
    if not isinstance(layout, list):
                      ^^^^^^
UnboundLocalError: cannot access local variable 'layout' where it is not associated with a value

Screenshots

Screen.Recording.2025-06-27.at.13.04.23.mov

Other

It looks like the bug comes from this PR and the following change (can't send the direct link to the code section as the PR introduced many changes):