GUI: start progress bar from background

The progress bar can only be updated after
the thread has started, so show it from
the thread.

java/org/contikios/cooja/GUI.java

@@ -1363,9 +1363,16 @@ public SwingWorker<Simulation, Cooja.SimulationCreationException> createLoadSimW
     // SwingWorker can pass information from worker to process() through publish().
     // Communicate information the other way through this shared queue.
     final var channel = new SynchronousQueue<Integer>(true);
-    var worker = new SwingWorker<Simulation, Cooja.SimulationCreationException>() {
+    return new SwingWorker<>() {
       @Override
       public Simulation doInBackground() {
+        if (progressDialog != null) {
+          java.awt.EventQueue.invokeLater(() -> {
+            progressPanel.setVisible(true);
+            progressDialog.getRootPane().getDefaultButton().addActionListener(e -> cancel(true));
+            progressDialog.setVisible(true);
+          });
+        }
         Element root = configFile == null ? cooja.extractSimulationConfig() : null;
         boolean shouldRetry;
         Simulation newSim = null;
@@ -1473,15 +1480,6 @@ public void actionPerformed(ActionEvent e) {
         }
       }
     };
-
-    if (progressDialog != null) {
-      java.awt.EventQueue.invokeLater(() -> {
-        progressPanel.setVisible(true);
-        progressDialog.getRootPane().getDefaultButton().addActionListener(e -> worker.cancel(true));
-        progressDialog.setVisible(true);
-      });
-    }
-    return worker;
   }
 
   public void updateProgress(boolean stoppedSimulation) {