Resolve #3437381 "Ajax error undefined"

Closes #3437381

src/FormAlter.php

@@ -49,7 +49,7 @@ class FormAlter {
     // Only alter the block configuration forms for inline or reusable
     // block_content entity blocks, since they are primarily the ones afflicted
     // with the conflict described above.
-    $is_reusable_block_content = $form['settings']['provider']['#value'] === 'block_content';
+    $is_reusable_block_content = ($form['settings']['provider']['#value'] ?? '') === 'block_content';
     $is_inline_block = isset($form['settings']['block_form']['#block']) && $form['settings']['block_form']['#block'] instanceof BlockContent;
     if (!$is_reusable_block_content && !$is_inline_block) {
       return;