Fatal Error with WP_Error as array in Webmention Form

Filed an Issue pfefferle/wordpress-webmention (GitHub)

A Webmention plugin for WordPress https://wordpress.org/plugins/webmention/

As mentioned in IW chat:

When placing the URL into the Webmention Form at the following Fatal Error appears at
Fatal error: Uncaught Error: Cannot use object of type WP_Error as array in /htdocs/wp-content/plugins/webmention/templates/webmention-api-message.php:119 Stack trace: #0 /htdocs/wp-content/plugins/webmention/includes/class-webmention-receiver.php(161): require_once() #1 /htdocs/wp-includes/class-wp-hook.php(298): Webmention_Receiver::serve_request(false, Object(WP_REST_Response), Object(WP_REST_Request), Object(WP_REST_Server)) #2 htdocs/wp-includes/plugin.php(203): WP_Hook->apply_filters(false, Array) #3 /htdocs/wp-includes/rest-api/class-wp-rest-server.php(380): apply_filters('rest_pre_serve_...', false, Object(WP_REST_Response), Object(WP_REST_Request), Object(WP_REST_Server)) #4 /htdocs/wp-includes/rest-api.php(271): WP_REST_Server->serve_request('/webmention/1.0...') #5 /htdocs/wp- in /htdocs/wp-content/plugins/webmention/templates/webmention-api-message.php on line 119