Attach the reference axes to the real root node

src/main/java/fr/ill/ics/n3d/gui/EditorMenuBar.java

@@ -2921,19 +2921,15 @@ public class EditorMenuBar extends MenuBar {
 		// Reference axis list display.
 		referenceAxisDisplay = new ReferenceAxisListDisplay();
 		
-		// Get the root node.
-		Group rootNode = (Group)Nomad3DEditor.getInstance().getModel().getRoot().getSceneNode();
-		
 		// We set it at the end of the group so that it is always in front of the components.
-		rootNode.getChildren().add(referenceAxisDisplay.getVisualGroup());
+		Nomad3DEditor.getInstance().getRoot().getChildren().add(referenceAxisDisplay.getVisualGroup());
 	}
 	
 	private void removeReferenceAxes() {
 		
 		if (referenceAxisDisplay != null && Nomad3DEditor.getInstance().getModel() != null) {
-			// Get the root node and remove the display.
-			Group rootNode = (Group)Nomad3DEditor.getInstance().getModel().getRoot().getSceneNode();
-			rootNode.getChildren().remove(referenceAxisDisplay.getVisualGroup());
+			// Remove the display.
+			Nomad3DEditor.getInstance().getRoot().getChildren().remove(referenceAxisDisplay.getVisualGroup());
 			referenceAxisDisplay = null;
 		}
 	}