Delete head branch causes "no ref found" error

[ZIJ]

if we use the delete head branch feature of github.
After digger merge the PR, we get this comment posted
TriggerDiggerJobs error: failed to trigger github workflow, POST https://api.github.com/repos/OWNER/REPO/actions/workflows/digger_workflow.yml/dispatches: 422 No ref found for: deleted_branch name []

Reported by user A.T.