while adding tab, i am getting this exception, how can i solve this? - gwt

Question

public void addTab(String text, Widget widget) {
TabItem item = new TabItem();
item.setText(text);
item.setClosable(true);
//item.add((Widget) new Dashboard());
tabPanel.add(item);
tabPanel.setSelection(item);
}
While Executing i am getting this exception,
how can i solve this problem in gxt. i am trying to add tab, but it gives this exception
com.google.gwt.event.shared.UmbrellaException: Exception caught: null
at com.google.gwt.event.shared.HandlerManager.fireEvent(HandlerManager.java:129)
at com.google.gwt.user.client.ui.Widget.fireEvent(Widget.java:129)
at com.sencha.gxt.widget.core.client.Component.fireEvent(Component.java:293)
at com.sencha.gxt.widget.core.client.button.IconButton.onClick(IconButton.java:215)
at com.sencha.gxt.widget.core.client.button.IconButton.onBrowserEvent(IconButton.java:196)
at com.google.gwt.user.client.DOM.dispatchEventImpl(DOM.java:1351)
at com.google.gwt.user.client.DOM.dispatchEvent(DOM.java:1307)
at sun.reflect.GeneratedMethodAccessor44.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at com.google.gwt.dev.shell.MethodAdaptor.invoke(MethodAdaptor.java:103)
at com.google.gwt.dev.shell.MethodDispatch.invoke(MethodDispatch.java:71)
at com.google.gwt.dev.shell.OophmSessionHandler.invoke(OophmSessionHandler.java:172)
at com.google.gwt.dev.shell.BrowserChannelServer.reactToMessagesWhileWaitingForReturn(BrowserChannelServer.java:338)