Some minor adjustments that localize the problem of GUI in fxlm file itself (root not correctly set?) I moved the fxml file to a spot, where gradle expects it

build.gradle

@@ -1,6 +1,7 @@
 plugins {
     id 'java-library'
     id 'application'
+    id 'org.beryx.jlink' version '2.12.0'
     id 'org.openjfx.javafxplugin' version '0.0.10'
     id 'jacoco'
 }
@@ -31,8 +32,8 @@ dependencies {
     implementation 'org.apache.logging.log4j:log4j-api:2.17.1'
     implementation 'org.apache.logging.log4j:log4j-core:2.17.1'
     implementation 'org.openjfx:javafx-controls:18'
-    implementation group: 'org.apache.commons', name: 'commons-collections4', version: '4.4'
+    implementation 'org.apache.commons:commons-collections4:4.4'
-    implementation group: 'org.openjfx', name: 'javafx', version: '11', ext: 'pom'
+    implementation 'org.openjfx:javafx:19-ea+5'
     testImplementation('org.junit.jupiter:junit-jupiter:5.8.2')
 }
 

src/main/java/ch/unibas/dmi/dbis/cs108/multiplayer/client/gui/chat/ChatApp.java

@@ -1,21 +1,21 @@
 package ch.unibas.dmi.dbis.cs108.multiplayer.client.gui.chat;
 
 import ch.unibas.dmi.dbis.cs108.multiplayer.client.gui.ClientModel;
-import java.io.BufferedReader;
+import java.net.URL;
-import java.io.InputStream;
+import java.util.Objects;
-import java.io.InputStreamReader;
 import javafx.application.Application;
 import javafx.fxml.FXMLLoader;
 import javafx.scene.Parent;
 import javafx.scene.Scene;
-import javafx.scene.layout.AnchorPane;
 import javafx.stage.Stage;
 
 public class ChatApp extends Application {
   ClientModel clientModel;
   private ChatController chatController;
 
-
+  public ChatApp() {
+    super();
+  }
   public ChatApp(ClientModel clientModel) {
     this.clientModel = clientModel;
     this.chatController = new ChatController(clientModel);
@@ -36,14 +36,25 @@ public class ChatApp extends Application {
    */
   @Override
   public void start(Stage primaryStage) throws Exception {
-
+    URL resource = ChatApp.class.getResource(
+        "splitPaneChatView.fxml");
+    if (resource == null) {
+      System.out.println("File wasnt found");
+    }
     //ChatApp chatApp = new ChatApp(new ClientModel());
-    Parent root = FXMLLoader.load(getClass().getResource("splitPaneChatView.fxml"));
+    try {
-    // TODO bin chatController.getChatPaneRoot() border to root border for rezising
+      Parent root = FXMLLoader.load(
-    Scene scene = new Scene(root);
+          Objects.requireNonNull(ChatApp.class.getResource(
+              "splitPaneChatView.fxml")));
+      // TODO bin chatController.getChatPaneRoot() border to root border for rezising
+      Scene scene = new Scene(root);
+      scene.setRoot(root);
+      primaryStage.setScene(scene);
+    } catch (Exception e) {
+      e.printStackTrace();
+    }
     primaryStage.setResizable(true);
     primaryStage.setTitle("Chat");
-    primaryStage.setScene(scene);
     primaryStage.show();
 
 

src/main/java/ch/unibas/dmi/dbis/cs108/multiplayer/client/gui/chat/ChatController.java

@@ -51,7 +51,9 @@ public class ChatController implements Initializable {
   private static final String chatToAll = Protocol.chatMsgToAll;
   private static final String chatToLobby = Protocol.chatMsgToLobby;
 
-
+  public ChatController() {
+    super();
+  }
   public ChatController(ClientModel client) {
     this.client = client;
     whisperTargetChosen = new SimpleBooleanProperty();

src/main/resources/ch/unibas/dmi/dbis/cs108/multiplayer/client/gui/chat/splitPaneChatView.fxml

@@ -0,0 +1,67 @@
+<?xml version="1.0" encoding="UTF-8"?>
+
+
+<?import javafx.geometry.Insets?>
+<?import javafx.scene.control.Button?>
+<?import javafx.scene.control.ScrollPane?>
+<?import javafx.scene.control.SplitPane?>
+<?import javafx.scene.control.TextArea?>
+<?import javafx.scene.control.TextField?>
+<?import javafx.scene.layout.AnchorPane?>
+<?import javafx.scene.layout.VBox?>
+
+
+<fx:root maxHeight="-Infinity" maxWidth="-Infinity" minHeight="-Infinity" minWidth="-Infinity" prefHeight="400.0" prefWidth="600.0" type="AnchorPane" xmlns="http://javafx.com/javafx/18" xmlns:fx="http://javafx.com/fxml/1" fx:controller="ch.unibas.dmi.dbis.cs108.multiplayer.client.gui.chat.ChatController">
+   <children>
+      <SplitPane fx:id ="chatPaneRoot" dividerPositions="0.5" layoutX="214.0" layoutY="92.0" orientation="VERTICAL" prefHeight="400.0" prefWidth="600.0" AnchorPane.bottomAnchor="0.0" AnchorPane.leftAnchor="0.0" AnchorPane.rightAnchor="0.0" AnchorPane.topAnchor="0.0">
+        <items>
+          <AnchorPane minHeight="0.0" minWidth="0.0" prefHeight="100.0" prefWidth="160.0">
+               <children>
+                  <ScrollPane layoutX="149.0" layoutY="-18.0" prefHeight="196.0" prefWidth="598.0" AnchorPane.bottomAnchor="0.0" AnchorPane.leftAnchor="0.0" AnchorPane.rightAnchor="0.0" AnchorPane.topAnchor="0.0">
+                     <content>
+                        <VBox fx:id="vBoxChatMessages" prefHeight="200.0" prefWidth="581.0" />
+                     </content>
+                  </ScrollPane>
+               </children>
+            </AnchorPane>
+          <AnchorPane minHeight="0.0" minWidth="0.0" prefHeight="100.0" prefWidth="160.0">
+               <children>
+                  <SplitPane dividerPositions="0.29797979797979796" layoutX="166.0" layoutY="8.0" prefHeight="195.0" prefWidth="598.0" AnchorPane.bottomAnchor="0.0" AnchorPane.leftAnchor="0.0" AnchorPane.rightAnchor="0.0" AnchorPane.topAnchor="0.0">
+                    <items>
+                      <AnchorPane minHeight="0.0" minWidth="0.0" prefHeight="160.0" prefWidth="100.0">
+                           <children>
+                              <SplitPane dividerPositions="0.5" layoutY="-3.0" orientation="VERTICAL" prefHeight="193.0" prefWidth="174.0" AnchorPane.bottomAnchor="0.0" AnchorPane.leftAnchor="0.0" AnchorPane.rightAnchor="0.0" AnchorPane.topAnchor="0.0">
+                                <items>
+                                  <AnchorPane minHeight="0.0" minWidth="0.0" prefHeight="100.0" prefWidth="160.0">
+                                       <children>
+                                          <Button fx:id="sendButton" layoutX="50.0" layoutY="21.0" mnemonicParsing="false" prefHeight="92.0" prefWidth="172.0" text="Send" AnchorPane.bottomAnchor="0.0" AnchorPane.leftAnchor="0.0" AnchorPane.rightAnchor="0.0" AnchorPane.topAnchor="0.0" />
+                                       </children>
+                                    </AnchorPane>
+                                  <AnchorPane minHeight="0.0" minWidth="0.0" prefHeight="100.0" prefWidth="160.0">
+                                       <children>
+                                          <TextField fx:id="whisperTargetSelectField" layoutY="14.0" prefHeight="92.0" prefWidth="172.0" promptText="whisper..." AnchorPane.bottomAnchor="0.0" AnchorPane.leftAnchor="0.0" AnchorPane.rightAnchor="0.0" AnchorPane.topAnchor="0.0" />
+                                       </children>
+                                    </AnchorPane>
+                                </items>
+                              </SplitPane>
+                           </children>
+                           <padding>
+                              <Insets bottom="5.0" left="5.0" top="3.0" />
+                           </padding>
+                        </AnchorPane>
+                      <AnchorPane minHeight="0.0" minWidth="0.0" prefHeight="160.0" prefWidth="100.0">
+                           <children>
+                              <TextArea fx:id="chatMsgField" layoutX="120.0" layoutY="-30.0" prefHeight="193.0" prefWidth="415.0" AnchorPane.bottomAnchor="0.0" AnchorPane.leftAnchor="0.0" AnchorPane.rightAnchor="0.0" AnchorPane.topAnchor="0.0" />
+                           </children>
+                           <padding>
+                              <Insets bottom="5.0" right="5.0" />
+                           </padding>
+                        </AnchorPane>
+                    </items>
+                  </SplitPane>
+               </children>
+            </AnchorPane>
+        </items>
+      </SplitPane>
+   </children>
+</fx:root>