Issue is with manipulating the $extraData. The GridFieldDetailForm_ItemRequest get that data by customising the $data['ManyMany'] but betterbuttons is not like that.
I think try{} can be done like (code snippet from GridFieldDetailForm_ItemRequest::doSave()):
When you have pushed a many many extra field to the GridField detail form and hit the
Save and close
button it triggers an errorIssue is with manipulating the
$extraData
. TheGridFieldDetailForm_ItemRequest
get that data by customising the$data['ManyMany']
but betterbuttons is not like that.I think
try{}
can be done like (code snippet fromGridFieldDetailForm_ItemRequest::doSave()
):It'll be starting point to fix the issue.