Thunderbird Extension Development, Log Error Message "Security Error: Content at moz-nullprincipal"

Question

Hello Stackoverflow community,

I am facing a small problem with Thunderbird, more specifically with the development of an extension. The developed extension should automatically forward the selected email to a predefined email by clicking on the popup icon on the right. In the process, additional information is added to the body of the email to be forwarded (i.e. additional information + email body), which the client to be received needs in order to process it in a further process.

This works fine, but the images contained in the email to be forwarded are no longer displayed with the following error message:

Security-Error from TB-Log

This error always occurs as soon as an attempt is made to manipulate the e-mail body in any way or to manipulate it in the forwarding process. Here is a small code snippet:

let composeFwdMsg = await browser.compose.setComposeDetails(composeTab.id, {
  body: emailDetails.body,
});

More information about setComposeDetails()

A manual forwarding works without any problems and also sends the e-mail correctly (with all banners and additional attachments).

Many thanks in advance for your tips and help :-)