The method bind to Window.onload event will be fired when closing the sub-w

I am new to vaadin and now I'm facing this problem which I couldn't find any solution from any other topics or forums!

I have a bunch of code like this:

First, I have method to launch a sub-window

public static void previewFlowProcess(String procInstCode, Environment env) {
  Window window = new ProcessComponent(procInstCode, env);
  UI.getCurrent().addWindow(window); 
}

Then, the constructor of the Sub-window object is like following:

public ProcessComponent(String processInstCode, Environment env){
  this.browser = new BrowserFrame("Browser", new ExternalResource("http://localhost:8181/activiti-explorer/diagram-viewer/Test.jsp"));
  browser.setWidth("950px");
  browser.setHeight("600px");
  bulidComponent();
}

So it will create a BrowserFrame to open an external resource, then while building the sub-window,  the BrowserFrame will embeded to the sub-window.

Third, it's the code of method buildComponent()

private void bulidComponent() { 
 setCaption("Test"); 
 setContent(browser); 
 center(); 
 setModal(true); 
 setSizeUndefined(); 
}

And the external resource is a JSP with a method which will be called when window.onload event was captured:

<script>
window.onload = function () {
  alert("call method")
}
</script>

My problem is that when I close the sub-window, the method bind to window.onload event will be called again, which means in this case: the "call method" message will be alerted again when I close the sub-window.

Does anyone here could give me some advises? Thanks a lot!!