I've installed the TFS 2013 MultiValue control locally and it worked well in MTM.
However, I tried to open a Bug while running a test and in the Bug work item (where I have 2 multi value fields) one of the controls work ed well but the other control was grayed out with the message "Object reference not set To n Instance of an object".
After that I tried to open a Bug in MTM (which work ed well before) and both controls were grayed out with the same error message.
After restarting MTM the controls were working well in regular bug creation process but the issue in the second control while creating a Bug during a Test Run happened again and I had to restart MTM once again.
Can you please advise?