I'm new to Nifty GUI, so I'm following a tutorial here for making popups. For now, I'm just trying to get a very basic "test" popup to show, but I get multiple errors and none of them make much sense. To show a popup, I believe it is necessary to first have a Nifty Screen already showing, which I do. So here is the ScreenController for the working Nifty Screen:

public class WorkingScreen extends AbstractAppState implements ScreenController {

//Main is my jme SimpleApplication
private Main app;
private Nifty nifty;
private Screen screen;

public WorkingScreen() {}

public void equip(String slotstr) {
    int slot = Integer.valueOf(slotstr);
    System.out.println("Equipping item in slot "+slot);
    //Here's where it STOPS working.
    app.getPlayer().registerPopupScreen(nifty);
    System.out.println("Registered new popup");
    Element ele = nifty.createPopup(app.getPlayer().POPUP);
    System.out.println("popup is " +ele);
    nifty.showPopup(nifty.getCurrentScreen(), ele.getId(), null);
}

@Override
public void initialize(AppStateManager stateManager, Application app) {
    super.initialize(stateManager, app);
    this.app = (Main)app;
}

@Override
public void update(float tpf) { 
    /** jME update loop! */
}

public void bind(Nifty nifty, Screen screen) {
    this.nifty = nifty;
    this.screen = screen;
}

When I call equip(0) the system prints Equipping item in slot 0, then a lot of errors and none of the subsequent println()'s. Clearly it botches somewhere in Player.registerPopupScreen(Nifty nifty). Here's the method:

public final String POPUP = "Test Popup";
public void registerPopupScreen(Nifty nifty) {
    System.out.println("Attempting new popup");
    PopupBuilder b = new PopupBuilder(POPUP) {{
            childLayoutCenter();
            backgroundColor("#000a");

            panel(new PanelBuilder() {{
                id("List");
                childLayoutCenter();
                height(percentage(75));
                width(percentage(50));
                control(new ButtonBuilder("TestButton") {{
                    label("TestButton");
                    width("120px");
                    height("40px");
                    align(Align.Center);
                }});
            }});
    }};
    System.out.println("PopupBuilder success.");
    b.registerPopup(nifty);
    System.out.println("Registerpopup success.");
 }

Because that first println() doesn't show, it looks like this method isn't even called at all!

Edit

After removing all calls on the Player object, the popup works. It seems I'm not "allowed" to access the player from the ScreenController. Unfortunate, since I need information on the player for the popup. Is there a workaround?