Purpose
=======
As we try to open a mail.compose.message on a model that doesn't inherit
from mail.thread (Mimic a cowboy yellin' "Yihaaa" while shooting into the air
with one revolver in each hand).
Specification
=============
The only way to fix this issue while keeping the feature (Open a
mail composer) is to open the mail composer in mass_mail mode and
to remove the model on which it should be called.
The template won't be rendered in the mail composer but will
be rendered when sending the mass_mailing obviously, while avoiding
the unavoidable traceback when clicking on the 'Send' button
closesodoo/odoo#28849
Yannick Tivisse
602f7ea0bc