Fixed error when adding block while no block categories are defined

Closes #3482744

src/Controller/BrowserController.php

@@ -214,7 +214,7 @@ class BrowserController extends ControllerBase {
           $block_categories[$bundle_label]['links'] = $block_links;
           $block_categories[$bundle_label]['#type'] = 'details';
           $block_categories[$bundle_label]['#attributes']['class'][] = 'js-layout-builder-category';
-          $block_categories[$bundle_label]['#open'] = $blockcat->getOpened();
+          $block_categories[$bundle_label]['#open'] = isset($blockcat) ? $blockcat->getOpened() : TRUE;
           $block_categories[$bundle_label]['#title'] = $this->t('Reusable @block_type_label', ['@block_type_label' => $bundle_label]);
         }
       }