Hello Kashif,

 Thank you for your post. I have been looking into it and have created a small sample for you. In the sample I create an instance of the ToolWindow, the popup window that shows up  when the FieldChooserButton is pressed, and set a Closed event for this window. I set this event in the FieldChooserOpening event. Please find the attached sample and fill free to contact us if you have any further questions.