if that doesn't work, spotlight "finder" and look for the .plist file. delete it (but don't empty the trash). then you may need to logoff and on or restart to have finder recreate it's preferences. if that works, feel free to empty the trash. if that seems to cause more problems (which it really shouldn't), then replace the new .plist with the one in the trash.
were you able to fix this yet? If not go to ~/library/preferences and find com.apple.sidebar.plst. open com.apple.sidebar.plst with omni outliner. click the arrow next to useritmes to un-colaps it. then click the aroww by customlistitems. click the arrow next to each number, and down by the bottom it will say name and say what it is. when you get the the # where it says untitled cd delete that whole entry (the entire number) save. restart. I hope this helps.