PopupPanel doesnt seem to work

My code is this:
AsyncCallback<String> callback = new AsyncCallback<String>() {
public void onFailure(Throwable caught) {

Window.alert("async failed!");
}

public void onSuccess(String reply) {
//Window.alert("async success! We have the network info!");

PopupPanel popup = new PopupPanel(true);
popup.setWidget(new Label("sample info lalala"));
popup.show(); //??
popup.setPopupPosition(100,100);
//Window.alert("a pop up should have appeared!");

}
};
serverCall.getUsername(callback);

The pop up panel never shows... I dont understand.. should I add it
somewhere in the RootPanel? If I do, it ceases to be a pop up.

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