[IMP] Raise UserError when we try to convert a bad html text in plain text in html2plaintext.
[FIX] When composing an email without selecting a model_id(User, Discussion group, Partner, ...), you can save your email as new template (by default the model_id of 'mail.group'(Discussion group)).
[IMP] When you try to post an empty message, you can retry by clicking on send button without refreshing the page.