Fix minor error when popping the last scene

src/main/java/envoy/client/ui/SceneContext.java

@@ -13,6 +13,10 @@ import envoy.client.event.ThemeChangeEvent;
 import envoy.event.EventBus;
 
 /**
+ * Manages a stack of scenes. The most recently added scene is displayed inside
+ * a stage. When a scene is removed from the stack, its predecessor is
+ * displayed.<br>
+ * <br>
  * Project: <strong>envoy-client</strong><br>
  * File: <strong>SceneContext.java</strong><br>
  * Created: <strong>06.06.2020</strong><br>
@@ -75,8 +79,10 @@ public final class SceneContext {
 	 */
 	public void pop() {
 		sceneStack.pop();
-		stage.setScene(sceneStack.peek());
-		applyCSS();
+		if (!sceneStack.isEmpty()) {
+			stage.setScene(sceneStack.peek());
+			applyCSS();
+		}
 		stage.show();
 	}
 

src/main/java/envoy/client/ui/Startup.java

@@ -123,6 +123,7 @@ public final class Startup extends Application {
 		final var sceneContext = new SceneContext(stage);
 		sceneContext.load(SceneContext.SceneInfo.CHAT_SCENE);
 		sceneContext.<ChatSceneController>getController().initializeData(sceneContext, localDB, client, writeProxy);
+
 		stage.setTitle("Envoy");
 		stage.setMinHeight(400);
 		stage.setMinWidth(350);
@@ -137,7 +138,7 @@ public final class Startup extends Application {
 	 * {@inheritDoc}
 	 */
 	@Override
-	public void stop() throws Exception {
+	public void stop() {
 		try {
 			// Save Settings and PersistentLocalDB on shutdown
 			logger.info("Closing connection...");