I ran into this same issue. The update from #5 above fixed the error (although it was line 566 for me - not sure if thats just from a different version of the module or a typo).
In Drupal 10.4 this issue remains, but it seems like it is a UI / label issue. The field conditions are available under "Content field type" (not "Entity field").
I'm just adding this comment here in case it will help the next person who searches for this problem (and ends up on this issue).
The following (similar) warning appears in if a newline appears at the end of the last item in the "Breadcrumb path":
Warning: Uninitialized string offset 0 in Drupal\custom_breadcrumbs\Form\CustomBreadcrumbsForm->validateForm() (line 262 of modules/contrib/custom_breadcrumbs/src/Form/CustomBreadcrumbsForm.php).
The configuration cannot be saved until the last newline is deleted.
- Drupal 10.4.6
- Custom Breadcrumb 1.1.2
I'm seeing the same issue with six messages (`This page is configured to apply "Page redirect" Rabbit Hole action`) displaying for content (using a token to redirect to a field value: `[node:field_EXAMPLE_link:uri]`).
Using version: `version: '2.0.0-beta1'`
I could not get #7 to work right for my use case. However, an alternate method using hook_views_post_execute() DID work for me:
https://www.drupal.org/project/field_collection_views/issues/2075593#com... →
I post this here only in case others ran into circumstances where #7 did not work.