What are the steps to reproduce your issue?

Launch a new website.  

Edit / Update header, background image, and other text / Save

What is the current behavior that you observe?

When hitting save, a window appears:

Confirmation

If you discard the current edits, all unsaved changes will be lost. You can cancel to return to edit mode.

Hit cancel and exception appears:  

Odoo Client Error

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An error occurred

Please use the copy button to report the error to your support service.

See detailsUncaughtPromiseError

Uncaught Promise

UncaughtClientError > TypeError

Uncaught Javascript Error > Cannot read properties of null (reading 'isCollapsed')

What would be your expected behavior in this case?

Page should save without error.