This might not be the answer you are looking for, but I stripped back the Budgie Countdown applet to the bare basics of showing a popover, where you should be able to add your own widgets and code. It might help you translate your app to a budgie popover. I have it posted here (the part you want to pay attention to is the maingrid):
https://github.com/samlane-ma/budgie-empty-popover
I strongly recommend this link:
@fossfreedom and @vlijm have taught me more than they may realize from their applets alone…