Block Editor: Fix PHP warning showing up when loading editor styles while in RTL.

In RTL languages, WordPress adds style-editor-rtl.css editor styles to the global $editor_styles
This patch ignores handling these styles if the file is not preset.

Props desrosj, mostafa.s1990.
Fixes #45288.

Built from https://develop.svn.wordpress.org/branches/5.0@43923


git-svn-id: http://core.svn.wordpress.org/branches/5.0@43755 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/edit-form-blocks.php

@@ -198,12 +198,14 @@ if ( $editor_styles && current_theme_supports( 'editor-styles' ) ) {
 			}
 		} else {
 			$file = get_theme_file_path( $style );
+			if ( file_exists( $file ) ) {
 				$styles[] = array(
-				'css'     => file_get_contents( get_theme_file_path( $style ) ),
+					'css'     => file_get_contents( $file ),
 					'baseURL' => get_theme_file_uri( $style ),
 				);
 			}
 		}
+	}
 }
 
 // Image sizes.

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.0-beta5-43922';
+$wp_version = '5.0-beta5-43923';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.