OS X 10.10, iMessages crashing after upgrade
Surprisingly solution easy as 1.2.3.
Open Terminal it can be founded in Other folder in the Launchpad or in folder Utilities located in Applications.
Remove the following folders and/or files:
~/Library/Preferences anything with com.apple.ichat in the name
~/Library/Saved Application State/com.apple.iChat
Restart your Mac. Open iMessages and go to iMessages -> Preferences -> Accounts and check your account information. If it is necessary create account from scratch. All your messages should be recovered. It may take a lot of time due to db converting process.