为什么element.getElementsByTagName（）无法选择动态添加的节点？

final Button sendButton = new Button("Send");
sendButton.getElement().setId("button");
RootPanel.get().getElement().appendChild(sendButton.getElement());

NodeList buttonElement = Document.get().getElementsByTagName("button");
if(buttonElement != null && buttonElement.getLength() > 0){
buttonElement.getItem(0).setNodeValue("Changed");
}else{
GWT.log("Can't select button");
}
final Button sendButton = new Button("Send");
s