Error rendering buttons of the cell toolbar when deleting cells in a notebook

[jtpio]

Description

Using 4.0.0b1 on Binder can lead to the following errors being logged in the dev tools console:

Error rendering buttons of the cell toolbar:  TypeError: t.layout is null

Reproduce

• Create a notebook
• Create cells
• Select cells
• Delete them with the cut toolbar item

jupyterlab-4-delete-cells-dev-tools-error.mp4

Expected behavior

No error in the dev tools console.

Context

• JupyterLab 4.0.0b1 on Binder: https://gist.github.com/jtpio/4e9a3d435d7c3950dc4150c10abbb1d7

[krassowski]

Switching cell type produces a similar error:

Error rendering buttons of the cell toolbar:  TypeError: Cannot read properties of null (reading 'insertWidget')

or in Firefox:

Error rendering buttons of the cell toolbar:  TypeError: can't access property "insertWidget", t.layout is null

[JasonWeill]

May be fixed if #14139 is done

[JasonWeill]

As discussed at the JupyterLab weekly meeting, this is a bug we should fix in 4.0.x, but is not a release blocker for 4.0.0.

[krassowski]

I encountered something similar while porting the vim extension; it appears that when cell type is being switch the activeCellChanged signal emits with the old cell which is already disposed. It should not do that IMO.