MDL-52253 scales: Prevent deletion of the last global scale

lib/grade/grade_scale.php

@@ -258,13 +258,22 @@ public static function fetch_all_local($courseid) {
         return grade_scale::fetch_all(array('courseid'=>$courseid));
     }
 
+    /**
+     * Checks if this is the last scale on the site.
+     *
+     * @return bool
+     */
+    public function is_last_global_scale() {
+        return ($this->courseid == 0) && (count(self::fetch_all_global()) == 1);
+    }
+
     /**
      * Checks if scale can be deleted.
      *
      * @return bool
      */
     public function can_delete() {
-        return !$this->is_used();
+        return !$this->is_used() && !$this->is_last_global_scale();
     }
 
     /**