change String? to String for UrlRequest

Author: Alexander-Ordina

lib/src/chatbox.dart

@@ -56,10 +56,10 @@ class MessageActionEvent {
 enum UrlNavigationAction { allow, deny }
 
 class UrlNavigationRequest {
-  final String? url;
+  final String url;
 
   UrlNavigationRequest(
-    this.url
+    this.url,
   );
 }