"WebExtension context not found!" in TB 102.0b7 Beta

Hi. Our regimail AddOn runs fine until TB 99 Beta. But with 102.0b7 it does not work any more (OpenSUSE 15.3).

The AddOn appears and shows its buttons. But trying to call any functionality it does nothing and the error console only logs this:

WebExtension context not found! ExtensionParent.jsm:1165
    getContextById resource://gre/modules/ExtensionParent.jsm:1165
    recvAPICall resource://gre/modules/ExtensionParent.jsm:1005
    recvAPICall self-hosted:1115
    _recv resource://gre/modules/ConduitsChild.jsm:82
    receiveMessage resource://gre/modules/ConduitsParent.jsm:450

The problem is, that I do not have any idea about whats wrong. The error message does not show any of my source code files as the source for the issue.

Where do I start looking for the error? Is there something known about AddOns in 102.0b7?

The current source code of the AddOn is on GitHub:

Any idea where to start? Thanks!

The best place to ask is here