Fix error when using a navigation block that returns an empty fallbac…

…k result (#56629)

* Fix fatal error when using a navigation block fallback that returns an empty result

* Fix get_inner_blocks_from_unstable_location function

* Use an empty WP_Block_List as a return value

* Add a unit test for get_inner_blocks_from_navigation_post

lib/compat/wordpress-6.5/class-wp-navigation-block-renderer.php

@@ -185,7 +185,7 @@ private static function get_inner_blocks_html( $attributes, $inner_blocks ) {
 	private static function get_inner_blocks_from_navigation_post( $attributes ) {
 		$navigation_post = get_post( $attributes['ref'] );
 		if ( ! isset( $navigation_post ) ) {
-			return '';
+			return new WP_Block_List( array(), $attributes );
 		}
 
 		// Only published posts are valid. If this is changed then a corresponding change
@@ -214,7 +214,7 @@ private static function get_inner_blocks_from_fallback( $attributes ) {
 
 		// Fallback my have been filtered so do basic test for validity.
 		if ( empty( $fallback_blocks ) || ! is_array( $fallback_blocks ) ) {
-			return '';
+			return new WP_Block_List( array(), $attributes );
 		}
 
 		return new WP_Block_List( $fallback_blocks, $attributes );

packages/block-library/src/navigation/index.php

@@ -75,7 +75,7 @@ function block_core_navigation_sort_menu_items_by_parent_id( $menu_items ) {
 	function block_core_navigation_get_inner_blocks_from_unstable_location( $attributes ) {
 		$menu_items = block_core_navigation_get_menu_items_at_location( $attributes['__unstableLocation'] );
 		if ( empty( $menu_items ) ) {
-			return '';
+			return new WP_Block_List( array(), $attributes );
 		}
 
 		$menu_items_by_parent_id = block_core_navigation_sort_menu_items_by_parent_id( $menu_items );

phpunit/class-wp-navigation-block-renderer-test.php

@@ -62,4 +62,23 @@ public function test_gutenberg_get_markup_for_inner_block_site_title() {
 		$expected = '<li class="wp-block-navigation-item"><h1 class="wp-block-site-title"><a href="http://' . WP_TESTS_DOMAIN . '" target="_self" rel="home">Test Blog</a></h1></li>';
 		$this->assertEquals( $expected, $result );
 	}
+
+	/**
+	 * Test that the `get_inner_blocks_from_navigation_post` method returns an empty block list for a non-existent post.
+	 *
+	 * @group navigation-renderer
+	 *
+	 * @covers WP_Navigation_Block_Renderer::get_inner_blocks_from_navigation_post
+	 */
+	public function test_gutenberg_get_inner_blocks_from_navigation_post_returns_empty_block_list() {
+		$reflection = new ReflectionClass( 'WP_Navigation_Block_Renderer' );
+		$method     = $reflection->getMethod( 'get_inner_blocks_from_navigation_post' );
+		$method->setAccessible( true );
+		$attributes = array( 'ref' => 0 );
+
+		$actual   = $method->invoke( $reflection, $attributes );
+		$expected = new WP_Block_List( array(), $attributes );
+		$this->assertEquals( $actual, $expected );
+		$this->assertCount( 0, $actual );
+	}
 }