Editor: Cast the result of the default_content, default_title, an…

…d `default_excerpt` filters.

If a plugin returns a non-string value (or returns `null`) on these filters, it can cause errors in the block editor. Casting them as a string prevents these errors.

Merges [43858] from the 5.0 branch to trunk.

Props dd32.
See #45236.


git-svn-id: https://develop.svn.wordpress.org/trunk@44224 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/post.php

@@ -683,7 +683,7 @@ function get_default_post_to_edit( $post_type = 'post', $create_in_db = false )
 	 * @param string  $post_content Default post content.
 	 * @param WP_Post $post         Post object.
 	 */
-	$post->post_content = apply_filters( 'default_content', $post_content, $post );
+	$post->post_content = (string) apply_filters( 'default_content', $post_content, $post );
 
 	/**
 	 * Filters the default post title initially used in the "Write Post" form.
@@ -693,7 +693,7 @@ function get_default_post_to_edit( $post_type = 'post', $create_in_db = false )
 	 * @param string  $post_title Default post title.
 	 * @param WP_Post $post       Post object.
 	 */
-	$post->post_title = apply_filters( 'default_title', $post_title, $post );
+	$post->post_title = (string) apply_filters( 'default_title', $post_title, $post );
 
 	/**
 	 * Filters the default post excerpt initially used in the "Write Post" form.
@@ -703,7 +703,7 @@ function get_default_post_to_edit( $post_type = 'post', $create_in_db = false )
 	 * @param string  $post_excerpt Default post excerpt.
 	 * @param WP_Post $post         Post object.
 	 */
-	$post->post_excerpt = apply_filters( 'default_excerpt', $post_excerpt, $post );
+	$post->post_excerpt = (string) apply_filters( 'default_excerpt', $post_excerpt, $post );
 
 	return $post;
 }