Java 无法在层次结构[GWT]中的指定点插入节点

我创建了一个类似

public class FinalizeEvalutionReportPanel extends VerticalPanel{

    //other code...

    public class PopupPanel extends DialogBox
    {
        public PopupPanel() {
            this.setStyleName("gwt-DialogBoxNew");
            this.setText(WorkFlowUtil.getPropertyValue(propertyKeys.showAgreement));
            this.setWidth(StringUtil.getWidthByGivenPercentage(30)+"px");
            this.add(this);
            this.center();
            addHandler();
        }
        private void addHandler() {
            final DialogBox box=this;
            giveFeedbackBtn.addClickHandler(new ClickHandler() {

                @Override
                public void onClick(ClickEvent event) {
                    box.hide();
                }
            });
        }
    }

}

我试着叫它，从别的班上点击按钮

showAgreementBtn.addClickHandler(new ClickHandler() {

        @Override
        public void onClick(ClickEvent event) {
        new FinalizeEvalutionReportPanel().showPopUp();//this give Java Script error

        }
    });

我犯了这个错误

com.google.gwt.core.client.JavaScriptException:（NS_ERROR\u DOM\u HIERARCHY\u REQUEST\u ERR）@com.google.gwt.DOM.client.Node:：appendChild（Lcom/google/gwt/DOM/client/Node；）（[JavaScript object（7280）]：无法在层次结构中的指定点插入节点

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问题可能出在下一行：

this.add(this);

不能添加组件本身