Error thrown when removing the only cell

[krassowski]

Description

If there is only one cell in the notebook and it gets removed (which is a bit pathological edge case) an error is thrown:

TypeError: Cannot read property 'model' of undefined
    at Mt.activeCellChanged (237.a21e094affef50264d99.js:formatted:4176)
    at m (jlab_core.61e1fcd0f32a80614a78.js:2)
    at Object.l [as emit] (jlab_core.61e1fcd0f32a80614a78.js:2)
    at e.emit (jlab_core.61e1fcd0f32a80614a78.js:2)
    at ne.set activeCellIndex [as activeCellIndex] (jlab_core.61e1fcd0f32a80614a78.js:2)
    at ne.onCellRemoved (jlab_core.61e1fcd0f32a80614a78.js:2)
    at ne._removeCell (jlab_core.61e1fcd0f32a80614a78.js:2)
    at jlab_core.61e1fcd0f32a80614a78.js:2
    at c (jlab_core.61e1fcd0f32a80614a78.js:2)
    at ne._onCellsChanged (jlab_core.61e1fcd0f32a80614a78.js:2)

Users previously complained about uncaught errors (#543 and a few earlier) and it would be nice to handle this one too.

The important use case is when selecting all cells and removing all of them to clear notebook as the only cells gets removed too.

Importantly the removed cells is immediately re-added by the model (a notebook has to have at least one cell) but we are fast and apparently can get into this strange state of the model when cell was already removed but not yet re-added.

Reproduce

1. Open new notebook
2. Press esc followed by dd (d twice)
3. See error in console

Expected behavior

Be nice, do not throw errors.

Context

• JupyterLab version: 3.0.7
• Lab extension 3.4.1