There has for a long time been an issue that when you try to add a new activity to the toolbox when editing a TFS build workflow Visual Studio can crash. I have seen it many times and never got to the bottom of it. It seems to be machine specific, as one machine can work while another supposedly identical will fail, but I could never track down the issue.
Today I was on a machine that was failing, but …
But I found a workaround in a really old forum post. The workaround is to load the IDE from the command line with the /safemode flag
C:Program Files (x86)Microsoft Visual Studio 12.0Common7IDEdevenv.exe /safemode
Once you do this you can edit the contents of your toolbox without crashes, and also your template if you wish. The best part is that once you exit the IDE and reload it as normal your new toolbox contents are still there.
Not perfect, but a good workaround