Thanks, I tried removing the reference file, but it still has the same problem. If it helps, this is from the problem report message;
Process: Messages [345]
Path: /Applications/Messages.app/Contents/MacOS/Messages
Identifier: com.apple.iChat
Version: 7.0.1 (3322)
Build Info: iChat-3321000000000000~4
Code Type: X86-64 (Native)
Parent Process: launchd [153]
User ID: 501
Date/Time: 2015-04-30 11:17:49.104 +0100
OS Version: Mac OS X 10.8.5 (12F2518)
Report Version: 10
Interval Since Last Report: 21831 sec
Crashes Since Last Report: 8
Per-App Interval Since Last Report: 18 sec
Per-App Crashes Since Last Report: 8
Anonymous UUID: 4983EF0A-B0CE-6531-B7E4-BDB84F77E769
Crashed Thread: 0 Dispatch queue: com.apple.main-thread
Exception Type: EXC_CRASH (SIGABRT)
Exception Codes: 0x0000000000000000, 0x0000000000000000
Application Specific Information:
*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'NSConcreteMutableAttributedString addAttribute:value:range:: nil value'
terminate called throwing an exception
abort() called