Widgets cannot be displayed when the notebook is opened with no kernel attached, even after we attach the notebook to an ipykernel.

[Kontinuation]

trafficstars

Description

I know that ipywidgets does not render without a kernel and it is expected behavior (see https://github.com/jupyter-widgets/ipywidgets/issues/3773). However, after I attach ipywidget to the notebook and run cells that produces widgets, the widgets still does not show up.

Reproduce

Launch the jupyter lab using the following command:

jupyter lab --LabServerApp.notebook_starts_kernel=False

The --LabServerApp.notebook_starts_kernel=False option makes new notebooks being opened in "No Kernel" state. When you run a cell, Jupyter Lab will open a dialog asking you to select a kernel. We select "ipykernel" to run the cells, but the widgets produced by the cell does not show up.

Here is a screen recording of a repro: https://drive.google.com/file/d/1EHGwKnv7ehDmMGw_AC4VAnseUprJPAcl/view?usp=sharing

Expected behavior

After attaching the notebook to ipykernel, newly produced widgets could show up.

Context

• ipywidgets version: latest main branch (commit: https://github.com/jupyter-widgets/ipywidgets/commit/303cae4dc268640a01ce08bf6e22da6c5cd201e4)
• Operating System and version: macOS 15.2
• Browser and version: Chrome Version 131.0.6778.265 (Official Build) (arm64)

Browser Output

plugin.ts:153 Uncaught (in promise) TypeError: Cannot read properties of null (reading 'kernel')
    at getWidgetManagerOwner (plugin.ts:153:34)
    at async registerWidgetHandler (plugin.ts:175:25)
    at async plugin.ts:424:7

If using JupyterLab

• JupyterLab version: 4.3.3

Installed Labextensions

jupyter_spark_ui v0.1.0 enabled OK (python, jupyter_spark_ui)
jupyterlab_pygments v0.3.0 enabled OK (python, jupyterlab_pygments)
sparkmonitor v3.0.2 enabled OK (python, sparkmonitor)
@jupyter-notebook/lab-extension v7.3.1 enabled OK
@jupyter-widgets/jupyterlab-manager v5.0.13 enabled OK