Cast string values to arrays

packages/block-editor/src/store/selectors.js

@@ -1628,8 +1628,18 @@ const isBlockVisibleInTheInserter = (
 	checkedBlocks.add( blockName );
 
 	// If parent blocks are not visible, child blocks should be hidden too.
-	if ( Array.isArray( blockType?.parent ) ) {
-		return blockType.parent.some(
+	//
+	// In some scenarios, blockType.parent may be a string.
+	// A better approach would be sanitize parent in all the places that can be modified:
+	// block registration, processBlockType, filters, etc.
+	// In the meantime, this is a hotfix to prevent the editor from crashing.
+	const parent =
+		typeof blockType.parent === 'string' ||
+		blockType.parent instanceof String
+			? [ blockType.parent ]
+			: blockType.parent;
+	if ( Array.isArray( parent ) ) {
+		return parent.some(
 			( name ) =>
 				( blockName !== name &&
 					isBlockVisibleInTheInserter(
@@ -1643,6 +1653,7 @@ const isBlockVisibleInTheInserter = (
 				name === 'core/post-content'
 		);
 	}
+
 	return true;
 };