FormPanel error if insert to Gwt verticalPanel or contentPanel inside verticalPanel

[jimmy6]

I get the umbrella exception if i add the formpanel. Why?

VerticalPanel v = new VerticalPanel();

final FormPanel panel = new FormPanel();
panel.setHeading("Send SMS");
panel.setFrame(true);
panel.setSize(111, 111);

TextField tf = new TextField();
tf.setTitle("dfdf");
tf.render(v.getElement());

ContentPanel cp = new ContentPanel();
for (int i=0; i < 55; i++) {
Button bb = new Button(i + "");
cp.add(bb);
}
cp.add(panel);
cp.render(v.getElement());
cp.layout();//must call

spanElement.appendChild(v.getElement());

[jimmy6]

I can make it work by add and render.
panel.render(cp.getElement());

sendSmsPanel.render(cp.getElement());
cp.render(v.getElement());

cp.add(panel);
cp.add(sendSmsPanel);

... but all the components inside not functioning at all. validator not work, button not working...