While reading the code for ConfigDistroImportForm, I noticed a @todo in the buildForm method that asks:
// @todo why is this empty?
Answer why...
The reason $form_state->get('storage_comparer') is empty, is the parent class doesn't always add the storage comparer.
Early Return Conditions:
If empty($source_list) or !$storage_comparer->hasChanges(), the form is returned early, and the storage_comparer is not set in the form state.
If !$storage_comparer->validateSiteUuid(), the form is returned early, and the storage_comparer is not set in the form state.
See https://git.drupalcode.org/project/drupal/-/blob/11.x/core/modules/confi...
Updating the parent class by moving Line 173 to Line 125 fixed the issue for me.
Postponed
2.0
Code
This is probably best addressed as part of 📌 Explore alternatives to extending internal core ConfigSync form class Active .