Site editor: shift tab from the first block, or tab from the last block not working as expected

[talldan]

Description

I noticed that shift-tab (used to access the block toolbar) and tab (used to access the block inspector) aren't working as expected in the site editor. A JS error is being thrown:

Uncaught TypeError: Cannot destructure property 'tagName' of 'element' as it is undefined.
    at isFormElement (is-form-element.js:15:10)
    at HTMLBodyElement.onKeyDown (use-tab-nav.js:121:18)

Things seem to work ok in the post editor.

Step-by-step reproduction instructions

Shift-tabbing

1. Select the first block (usually a header template part, selecting via list view makes this easier)
2. Press shift tab

Expected - the toolbar is focused
Actual - nothing happens visually, a JS error is thrown

Tabbing

1. Add an image block as the last block and select it
2. Tab through the buttons in the placeholder and keep pressing tab

Expected - the block inspector is focused
Actual - nothing happens visually, a JS error is thrown

Screenshots, screen recording, code snippet

No response

Environment info

No response

Please confirm that you have searched existing issues in the repo.

Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

Yes