iCloud PreferencesPane crashes after upgrade to Yosemite
Solution 1:
iCloud preferences seem to be spread across a few different plist files. The main one (funnily enough) seems to be called /Users/YOURUSERNAME/Library/Preferences/MobileMeAccounts.plist. That's the one that stores your account details (ID, UUID, DSID and AlternateDSID) and information about the services you have enabled or disabled.
You could try closing System Preferences entirely, renaming that file temporarily, restarting your Mac and then seeing if System Preferences can then open its iCloud pane.