Issue #3367018: Fix warning for custom blocks

src/Controller/OverriddenBrowserController.php

@@ -64,7 +64,7 @@ class OverriddenBrowserController extends BrowserController {
 
       $links[] = $link;
       $pieces = explode(':', $block_id);
-      $bundle = $pieces[1];
+      $bundle = $pieces[1] ?? NULL;
       if ($block['id'] == 'inline_block') {
         $attributes = $this->getAjaxAttributes();
         $attributes['class'][] = 'browse-library-icon';