HTML POPUP "Invalid window handle"


I am using HTMLPOPUP since years in my projects. 

Now I have an issue with the component, without changing the code.

I have a modal-form with a button-click-event:

  s1 := "TEXT"

This works and I can click the button and show the popup as much as I like.

I leave the form (back to main form) and open it again. Now I get the error on the button-click: 

“System Error. Code: 1400. Invalid window handle”

Any idea? It worked for year. 

HTMLPopup / Delphi Seattle 

Thanks for help!

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Test code: