How to add MouseHandler in constructor? (NPE)

I know I can just bind any ui component to any events by using @UiHandler like this:

<g:FocusPanel ui:field="panel">

@UiField
FocusPanel panel;

@UiHandler("panel")
void handleClick(MouseDownEvent event) {
    //...
}

So far so good, but how can I "manually" add a mouse handler within the class constructor:
            panel.addMouseDownHandler(new MouseDownHandler() { //NPE
                @Override
                public void onMouseDown(MouseDownEvent event) {
                    Window.alert("mouse down");
                }
            });

If I write this in the constructor of the java class, I'm getting NullPointerException at this stage where the handler is added to the panel.
What am I missing here?

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