chore: Change url property String to nullable String? to make sure it will work with different implementations for the webview

Author: Alexander-Ordina

lib/src/chatbox.dart

@@ -56,7 +56,7 @@ class MessageActionEvent {
 enum UrlNavigationAction { allow, deny }
 
 class UrlNavigationRequest {
-  final String url;
+  final String? url;
 
   UrlNavigationRequest(
     this.url