I tried this already. Did not solve the problem. I was using CleanMyMac 3 to delete all the traces of the installation.
It seems to search for information (possibly in a plist file) which is not available.
Here is the error log:
Process: MainStage 3 [4315]
Path: /Applications/MainStage 3.app/Contents/MacOS/MainStage 3
Identifier: com.apple.mainstage3
Version: 3.2 (3982.43)
Build Info: MAMainStage-3982043000000000~2
App Item ID: 634159523
App External ID: 812006152
Code Type: X86-64 (Native)
Parent Process: ??? [1]
Responsible: MainStage 3 [4315]
User ID: 503
Date/Time: 2015-08-30 11:05:45.844 +0200
OS Version: Mac OS X 10.10.5 (14F27)
Report Version: 11
Anonymous UUID: AA973508-C481-14C1-51D3-838BD07844F1
Sleep/Wake UUID: B71A381F-25BE-43A5-BE8B-1A8D0828E04E
Time Awake Since Boot: 9900 seconds
Time Since Wake: 8800 seconds
Crashed Thread: 5 Dispatch queue: com.apple.root.default-qos
Exception Type: EXC_CRASH (SIGABRT)
Exception Codes: 0x0000000000000000, 0x0000000000000000
Application Specific Information:
*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** setObjectForKey: object cannot be nil (key: pkg-version)'
terminating with uncaught exception of type NSException
abort() called