Issue #3177797 by Adam Clarey, VladimirAus: Functional tests fail due to missing schema

bootstrap4.install

@@ -89,8 +89,8 @@ function bootstrap4_update_8103() {
 function bootstrap4_update_8220() {
   \Drupal::configFactory()
     ->getEditable('bootstrap4.settings')
-    ->clear('subtheme_folder', 'themes/custom')
-    ->clear('subtheme_name', '')
-    ->clear('subtheme_machine_name', '')
+    ->clear('subtheme_folder')
+    ->clear('subtheme_name')
+    ->clear('subtheme_machine_name')
     ->save(TRUE);
 }