I get this message : tinyMCE.get("message_data_" + msg_id) is undefined on Firebug after
I click on the button to submit the form.
msg_id is defined, I checked it. message_data_ is also defined.
The function tinyMCE.get is not for some reason.
If you are using one single editor instance you may use
tinymce.editors[0]instead oftinyMCE.get("message_data_" + msg_id).