[FIX] tiki-admin_structures.php: Ensured both structure and action are selected.
Ensured both structure and action are selected.
- Implemented error checks to ensure both structure and action are selected.
- Alerts with direct error messages: "Select structure", "No structure selected.", "No action selected."
Issue addressed in point 3 of the page: https://avan.tech/item112955-Tiki-structures-issues