Token click-insert does not insert tokens into "Custom body" of email handler

Problem/Motivation

In the email handler "Message" section, with Body set to "Custom body",
clicking a token in the token browser does not insert the token into the
CKEditor 5 body field. The token is not added to the text.
With Body set to "Twig template", the same click-insert works.
"Master" is read-only.

Steps to reproduce

1. Edit a webform and add an Email handler.
2. In "Message", set "Body" to "Custom body".
3. Open the token browser.
4. Place the cursor in the body field.
4.Click any token in the token browser e.g. [webform_submission:created].
Expected: the token is inserted at the cursor position.
Actual: nothing is inserted into the body field.

See the attached screen recording

Proposed resolution

Suspected cause (not yet verified)

The "Custom body" option appears to render several text fields, but only one
is visible at a time. The token click-insert routine seems to lack a reference
to the currently visible CKEditor instance, so it does not target the active field.

Remaining tasks

User interface changes

NONE

API changes

NONE

Data model changes

NONE

Disclaimer

Disclaimer: The author is not a native English speaker. An AI assistant (Claude) helped to formulate this issue description in English. The technical content, observations and reproduction steps are the author's own.