How to extend GWT Widgets and use them with Ui-Binder?

I'm creating a custom Tree component which extends the GWT Tree.
But how can I use my custom tree with ui:binder??

I tried the following which did not work:

my custom tree:
public class CustomTree extends Tree {
    public CustomTree() {
        //...
    }
}


MyPanel.ui.xml:
<ui:UiBinder>   
    <CustomTree ui:field="tree" />
</ui:UiBinder>


MyPanel.java:
public class MyPanel extends Composite {
    @UiField
    CustomTree tree;

    public MyPanel() {
        initWidget(binder.createAndBindUi(this));
        //...
    }
}

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