Fix fails in HEAD

Issue information

From user: larowlan

Related to !21 (merged)

Contribution record

Fork management

Problem/Motivation

Changes in Drupal core have caused failures in HEAD - looks like a return type change on a test method.
See https://git.drupalcode.org/project/aggregator/-/jobs/2617052

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

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