popup window could use an on_open event

**Is your feature request related to a problem? Please describe.**
For a single item, the popup works fine. However, when using a popup on a list of radio buttons, I would like to first determine which was selected then change the popup data accordingly.

**Describe the solution you'd like**
Create an on_open callback

**Describe alternatives you've considered**
Currently, I have to keep the popups updated throughout my methods versus just having one method to do the work.

**Additional context**
Thanks for your consideration