Cron form: leaving Session Key empty always fails, though the hint says "leave empty to use default"

Adding a cron task in the management UI with Session Key left empty fails with session_key is required, but the placeholder says 「留空使用默认」 ("leave empty to use default") — there is no default.

Checked the code: the backend requiring it is intentional (session_key is the routing anchor when the job fires). The frontend is the problem — misleading hint, no fallback, no validation; and a project with no sessions gets an empty dropdown, dead end.

Suggest: mark it as required, auto-select the most recent session when a project is picked, and validate before submit.