Resolve #3436620 "Invalidargumentexception please configure"

Closes #3436620

src/Controller/DropZones.php

@@ -421,7 +421,7 @@ class DropZones extends ControllerBase {
     // Insert a new section.
     if ($destination['type'] === 'section') {
       $layout_settings = $this->getLbPlusSetting($section_storage, 'default_section');
-      $layout_plugin_id = $layout_settings['layout_plugin'];
+      $layout_plugin_id = $layout_settings['layout_plugin'] ?? 'layout_twocol';
       $section = new Section($layout_plugin_id, $layout_settings);
       $section_uuid = $this->uuid->generate();
       $section->setThirdPartySetting('lb_plus', 'uuid', $section_uuid);