It works !

ChatApp/app/src/main/java/client/ClientLogin.java

@@ -5,9 +5,7 @@ import javafx.fxml.FXMLLoader;
 import javafx.scene.Scene;
 import javafx.scene.control.TextField;
 import javafx.stage.Stage;
-import client.ClientGui;
 import java.io.IOException;
-import java.net.InetSocketAddress;
 
 import static utilities.FxUtilities.centerStage;
 
@@ -21,10 +19,9 @@ public class ClientLogin {
         if(username.isEmpty()) {
             return;
         }
+        UsernameSingleton.getInstance().setUsername(username);
         var loader = new FXMLLoader(getClass().getResource("/client/clientLoading.fxml"));
-        Client client = new Client(new InetSocketAddress("localhost", 6665), loader.getController(), username);
         loader.load();
-        ((ClientLoading) loader.getController()).setClient(client);
         Stage stage = (Stage) usernameField.getScene().getWindow();
         stage.setScene(new Scene(loader.getRoot(), 800, 600));
         centerStage(stage);