Gradebook: Fix visibility change in gradebook links

Author: @BorjaSanchezBeezNest

main/gradebook/gradebook_edit_link.php

@@ -52,7 +52,7 @@ $form = new LinkAddEditForm(
     api_get_self().'?selectcat='.$linkcat.'&editlink='.$linkedit.'&'.api_get_cidreq()
 );
 if ($form->validate()) {
-    $values = $form->exportValues();
+    $values = $form->getSubmitValues();
     $parent_cat = Category::load($values['select_gradebook']);
     $final_weight = $values['weight_mask'];
     $link->set_weight($final_weight);