ya i had the same problem. if you do not want to reinstall, do this:
Download the tk-8.4-lastevent.patch from ,
place it in ur tk8.4.18 directory and go to the same directory and execute,
patch -p0 < ./tk-8.4-lastevent.patch
now reinstall tk. if it still does not work , reinstall the whole thing.
though this solution was for fedora, it worked for my ubuntu also.