There is a content location filter with null value.

The issue is caused by a bug in Sitefinity.

Fixed in:
Sitefinity Patch 15.4.8627.0

Backported in:
Sitefinity Patch 15.2.8440.0

Refer to Knowledge Base Article How to update Sitefinity to hotfix, internal build or a patch for instructions on how to download and install the fix.

The following SQL query can be used to check for other corrupted content location filters:

SELECT *
FROM sf_cnt_location_filters
WHERE value IS NULL

In case there are other corrupted entries, the following query can be used to determine the pages with widgets that have corrupted content location filters:

SELECT *
FROM sf_page_node
WHERE id IN (SELECT page_id
FROM sf_cnt_locations
WHERE id IN (
SELECT cnt_location_id
FROM sf_cnt_location_filters
WHERE value IS NULL))

To identify page locations for specific content type use the following query. Replace the highlighted type with the full type name of the content type needed:

SELECT * from sf_page_node
	WHERE id in(
	SELECT page_id
		FROM sf_cnt_locations
		WHERE id IN (
				SELECT cnt_location_id
				FROM sf_cnt_location_filters
				WHERE value IS NULL and type = 'Telerik.Sitefinity.ContentLocations.ContentLocationSingleItemFilter'
			) and item_type = 'Telerik.Sitefinity.DynamicTypes.Model.Tests.Test'
		)

If the problematic widget is dropped on a Page:

1. Identify the problematic page using the queries above.
2. Open the problematic page for editing
3. Duplicate the widget that is throwing the error
4. Publish the page
5. Remove the problematic widget and leave the duplicated widget
6. Publish the page

If the problematic widget is dropped on a Page template:

1. Identify the problematic page using the queries above.
2. Delete the page (Move to Recycle bin)*
3. Navigate to Dashboard -> Recycle bin
4. Restore the page

If the problematic page is a group page:

1. First delete the child pages  (Move to Recycle bin)*
2. Delete the parent page (the problematic one)
3. Navigate to Dashboard -> Recycle bin
4. Restore the parent page
5. Restore the child pages

*NOTE: Recycle bin must be enabled.