Fix: Potential fatal error if Block Compatibility is off (#1272)

* Fix undeclared class error

* Update docblock

---------

Co-authored-by: xxsimoxx <simone@gieffeedizioni.it>

src/wp-admin/includes/class-wp-screen.php

@@ -433,7 +433,9 @@ public function in_admin( $admin = null ) {
 	 * @return bool False.
 	 */
 	public function is_block_editor( ...$args ) {
-		WP_Compat::using_block_function();
+		if ( class_exists( 'WP_Compat' ) ) {
+			WP_Compat::using_block_function();
+		}
 		return false;
 	}
 

src/wp-includes/classicpress/class-wp-compat.php

@@ -190,6 +190,9 @@ public function delete_themes_using_blocks() {
 	 * This function have to be called from a polyfill
 	 * to map themes and plugins calling those functions.
 	 *
+	 * Make sure that class WP_Compat exists before calling the function
+	 * because it's not defined if Blocks Compatibility option is set to "off".
+	 *
 	 * @return void
 	 */
 	public static function using_block_function() {

src/wp-includes/post.php

@@ -7822,7 +7822,9 @@ function wp_untrash_post_set_previous_status( $new_status, $post_id, $previous_s
  * @return bool Whether the post can be edited in the block editor.
  */
 function use_block_editor_for_post( $post ) {
-	WP_Compat::using_block_function();
+	if ( class_exists( 'WP_Compat' ) ) {
+		WP_Compat::using_block_function();
+	}
 	return false;
 }
 
@@ -7839,6 +7841,8 @@ function use_block_editor_for_post( $post ) {
  * @return bool Whether the post type can be edited with the block editor.
  */
 function use_block_editor_for_post_type( $post_type ) {
-	WP_Compat::using_block_function();
+	if ( class_exists( 'WP_Compat' ) ) {
+		WP_Compat::using_block_function();
+	}
 	return false;
 }