fixed disappearing object when reopening simulator panel

src/gui/frames/MainFrame.java

@@ -23,7 +23,7 @@ public class MainFrame extends JFrame{
     public enum Views{EDITOR, SIMULATOR, AGENDA, TABLE};
     
     private Agenda agenda;
-    private JPanel currentPanel;
+    private JPanel currentPanel, simulatorpanel;
     
     private int length;
     private int terainwidth;
@@ -72,7 +72,12 @@ public class MainFrame extends JFrame{
                 this.updateView(new AgendaPane(this.agenda));
                 break;
             case SIMULATOR:
-                this.updateView(new SimulatorPane(this.agenda, this.length, this.terainwidth, this.terrain));
+            	if(simulatorpanel == null){
+            		simulatorpanel = new SimulatorPane(this.agenda, this.length, this.terainwidth, this.terrain);
+            		this.updateView(simulatorpanel);
+            	}else{
+            		this.updateView(simulatorpanel);
+            	}
                 break;
             case TABLE:
                 this.updateView(new Table(this.agenda));