fix: return empty object if selected mail driver is unavailable (#4113)

Author: DavideIadeluca

framework/core/js/src/admin/components/MailPage.tsx

@@ -98,7 +98,7 @@ export default class MailPage<CustomAttrs extends IPageAttrs = IPageAttrs> exten
 
   contentItems(): ItemList<Mithril.Children> {
     const items = new ItemList<Mithril.Children>();
-    const fields = this.driverFields![this.setting('mail_driver')()];
+    const fields = this.driverFields![this.setting('mail_driver')()] || {};
     const fieldKeys = Object.keys(fields);
 
     items.add(