The problem can be reproduced as follows:
1. login as admin and another use e.g. test1
2. start a IM conversation
3. user test1 invites admin to a chat
4. admin clicks on the invite button in the private chat and selects a third user, the error appears after the invite button is clicked
During this structured test I've found three additional bugs:
A: is user test1 is using Firefox for the invitation the private chat window for admin appears, but not for test1 (the invite window remains open after the invite button is pushed)
B: is user admin uses IE6 with Google PopUp blocker the "PopUp blocker" window appears after user test1 has finished his invitation, after clicking on "Click here to open..." a the "PopUp blocker detected" window appears a 2nd time and then a "Please choose a nickname window" is shown
C: the typing indicator is only shown in the Private Chat window and not in the IM window