sap.m.FlexBox is not rendering in OverlayContainer

Hello,

I'm working on a signature pad feature to my app using szimek/signature_pad. When a user presses "add signature" button, the overlay container pops up. This container is used to sign and accept/clear the signature.

The problem is that I want to align control buttons in a specific way using sap.m.FlexBox, but when I add my FlexBox object to the content of the overlay object, the FlexBox object is not rendered (but the myhtml part of the oOverlayContainer's content is rendered and works fine).

Here is the code of the relevant function: LINK.

What am I missing here?

Thanks!