"Send PM" button on post / profile not working

When trying to send PM to a user by clicking on "Send pm" in forum post or in profile.
Link redirect to correct url
private.php?action=send&uid=1

But uid is "undefined" in the new conversation recipient.
This error is also happening on mybboost.

Oops my bad, it is working anyway (but I thought the first time sending the message wasn't working o.O)

This is a noted bug that Shade said will be fixed in the next Beta...I noticed the same thing on my forum when I tested it.

This should be actually working in beta 2 as long as select2's option "multiple" is set to true; it's not here due to styling reasons. A definitive fix (which implies "multiple" to be always true) will be carried out in beta 3 which will support multiple participants in a single chat.