Form elements date_selector and datetime_selector initialize javascript in the constructor instead of when they are rendered.
This breaks the following scenarios for modal forms:

• Initialise an instance of the form
• Check if form was submitted,
• if yes
  • do something, redirect, return true.... whatever needs to be done. Except that form does not need to be rendered after it was submitted
• if not
  • start collecting javascript requirements
  • render form, capture output
  • return html and js

As a result the JS does not contain the js needed for the calendar icon in date selector.