Description

There's an error when saving widgets from editing a clean state in the new widgets screen. Found when writing tests in #28160.

Step-by-step reproduction instructions

1. Go to the new widgets screen.
2. Delete all widgets, including widgets in the inactive widgets section.
3. Save, reload, and confirm that there's no widgets left.
4. Add a new block (e.g. paragraph block) in the first widget area.
5. Toggle to open the second widget area (could be inactive widgets or others).
6. Add another block to the second widget area.
7. Hit "Update", see the error pops up in the snackbar.

Expected behaviour

There shouldn't be any errors.

Actual behaviour

There's an error and the widgets are not correctly saved.

Screenshots or screen recording (optional)

WordPress information

• WordPress version: 5.7-alpha-49956
• Gutenberg version: master branch
• Are all plugins except Gutenberg deactivated? Yes
• Are you using a default theme (e.g. Twenty Twenty-One)? Tested in both Twenty Twenty-One and Twenty Twenty

Device information

• Device: Desktop
• Operating system: macOS
• Browser: Chrome 87