uibinder widget add child fail

Question

i try to add child in uibinder

i get error

in a text-only context. Perhaps you are trying to use unescaped HTML where text is required, as in a HasText widget? Element <package:ListItem.....

my parent look like below

public class ListItem extends ComplexPanel implements HasText, HasHTML, HasClickHandlers, HasKeyDownHandlers, HasBlurHandlers {
    HandlerRegistration clickHandler;

    public ListItem() {
        setElement(DOM.createElement("LI"));
    }

    public void add(Widget w) {
        super.add(w, getElement());
    }

    public void insert(Widget w, int beforeIndex) {
        super.insert(w, getElement(), beforeIndex, true);
    }

    public String getText() {
        return DOM.getInnerText(getElement());
    }

    public void setText(String text) {
        DOM.setInnerText(getElement(), (text == null) ? "" : text);
    }

    public void setId(String id) {
        DOM.setElementAttribute(getElement(), "id", id);
    }

    public String getHTML() {
        return DOM.getInnerHTML(getElement());
    }

    public void setHTML(String html) {
        DOM.setInnerHTML(getElement(), (html == null) ? "" : html);
    }

    public HandlerRegistration addClickHandler(ClickHandler handler) {
        return addDomHandler(handler, ClickEvent.getType());
    }

    public HandlerRegistration addKeyDownHandler(KeyDownHandler handler) {
        return addDomHandler(handler, KeyDownEvent.getType());
    }

    public HandlerRegistration addBlurHandler(BlurHandler handler) {
        return addDomHandler(handler, BlurEvent.getType());
    }


}

can explain what the error mean?