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Follow-up of [#194362]
Quoting myself from there:
The new editor entity is only created on the ajax callback/configure button.
If you disable JS, add a new format, select an editor and don't click on "Configure" but directly on save, you get a "Fatal error: Call to undefined method stdClass::save() in .../core/modules/editor/editor.module on line 220 ".
The same will happen if you add a test where you leave out the drupalPostAjax() there.
I'm not sure what's the best thing to do, but you should be able to provoke fatal errors through the UI even if you're doing it "wrong" ;) Either add a validation along the lines of "You must configure the editor" and reload the form with the configuration thingy updated or create the editor entity on demand if it doesn't exist yet.