MDL-50569 backup: Missing get_target call

backup/moodle2/restore_stepslib.php

@@ -516,6 +516,7 @@ class restore_gradebook_structure_step extends restore_structure_step {
         $version28start = 2014111000.00;
         $version28last = 2014111006.05;
 
+        $target = $this->get_task()->get_target();
         if ($minmaxtouse === false &&
                 ($target != backup::TARGET_CURRENT_ADDING && $target != backup::TARGET_EXISTING_ADDING)) {
             // The setting was not found because this setting did not exist at the time the backup was made.