diff -r c7c34916027a -r 177826044cd9 wp/wp-admin/includes/class-wp-ajax-upgrader-skin.php
--- a/wp/wp-admin/includes/class-wp-ajax-upgrader-skin.php	Mon Oct 14 18:06:33 2019 +0200
+++ b/wp/wp-admin/includes/class-wp-ajax-upgrader-skin.php	Mon Oct 14 18:28:13 2019 +0200
@@ -61,8 +61,10 @@
 		$messages = array();
 
 		foreach ( $this->errors->get_error_codes() as $error_code ) {
-			if ( $this->errors->get_error_data( $error_code ) && is_string( $this->errors->get_error_data( $error_code ) ) ) {
-				$messages[] = $this->errors->get_error_message( $error_code ) . ' ' . esc_html( strip_tags( $this->errors->get_error_data( $error_code ) ) );
+			$error_data = $this->errors->get_error_data( $error_code );
+
+			if ( $error_data && is_string( $error_data ) ) {
+				$messages[] = $this->errors->get_error_message( $error_code ) . ' ' . esc_html( strip_tags( $error_data ) );
 			} else {
 				$messages[] = $this->errors->get_error_message( $error_code );
 			}
@@ -95,7 +97,7 @@
 
 			// Count existing errors to generate an unique error code.
 			$errors_count = count( $this->errors->get_error_codes() );
-			$this->errors->add( 'unknown_upgrade_error_' . $errors_count + 1 , $string );
+			$this->errors->add( 'unknown_upgrade_error_' . $errors_count + 1, $string );
 		} elseif ( is_wp_error( $errors ) ) {
 			foreach ( $errors->get_error_codes() as $error_code ) {
 				$this->errors->add( $error_code, $errors->get_error_message( $error_code ), $errors->get_error_data( $error_code ) );