slider wont work cause enabling the code to grow/shrink it makes it die

assignments/a2/src/main/java/Controls.java

@@ -19,16 +19,16 @@ public class Controls extends JPanel implements Observer
     this.setPreferredSize(new Dimension(1280,75));
     this.setMinimumSize(new Dimension(320, 75));
     this.playback = new JSlider(new DefaultBoundedRangeModel(0, 0, 0, 0));
-//    this.playback.addChangeListener(new ChangeListener()
-//    {
-//      public void stateChanged(ChangeEvent e)
-//      {
-//        if (playback.getValue() != model.getThingies().size())
-//	{
-//          model.changeThingy(playback.getValue() - 1);
-//	}
-//      }
-//    });
+    this.playback.addChangeListener(new ChangeListener()
+    {
+      public void stateChanged(ChangeEvent e)
+      {
+        if (playback.getValue() != model.getThingies().size())
+	{
+          model.changeThingy(playback.getValue() - 1);
+	}
+      }
+    });
     this.playback.setPreferredSize(new Dimension(280, 75));
     this.add(playback);
     model.addObserver(this);
@@ -36,8 +36,12 @@ public class Controls extends JPanel implements Observer
 
   public void update(Object observable)
   {
-    this.playback.setMaximum(model.getThingies().size());
-    this.playback.setValue(model.getThingies().size());
+    int len = model.getThingies().size();
+    if (len > 0)
+    {
+      //playback.setMaximum(len);
+      //playback.setValue(len);
+    }
     this.setBackground(model.controlColour);
     revalidate();
     repaint();