firstly, determine the current widget you want to interact with. This being your Close button.
do this like this -->
RS2Widget closebutton = getWidgets().get(firstchild, secondchild);
to get your firstchild and secondchild, use the widgetdebugger in the client and hover over the close button
then, once you've determined your widget, check if your widget is there/is visible.
do this like this -->
if (closebutton != null && closebutton.isVisible()){
}
now, you need to tell it to interact with that widget and its current available actions
This would be like this i guess, i'm not if its exactly "Close" so please check it yourself -->
closebutton.interact("Close");
Your final piece of code would be -->
RS2Widget closebutton = getWidgets().get(firstchild, secondchild);
if (closebutton != null && closebutton.isVisible()){
closebutton.interact("Close");
}