Crash at unknown git revision

1. In a git repository add a commit with a commit message that contains a hex-string which does not reference a commit in the repository.
   Sounds more complicated than it is, have a log like this:

   commit 62b256ad728402cb6170b54e018ec514969b61fd (HEAD, 3.5)
   Author: Robert Loehning <robert.loehning@theqtcompany.com>
   Date:   Wed Sep 9 17:09:18 2015 +0200
   
       Test commit
       
       This is 506c0685a3b78d3ea3c7eceeec48303e071c41b4 + 1 so it
       should be 506c0685a3b78d3ea3c7eceeec48303e071c41b5
       
       Change-Id: I8a32c5185d16d50b44b2b15c189d1a284b8fbf72
   
   commit 506c0685a3b78d3ea3c7eceeec48303e071c41b4 (origin/3.5)
   Author: Robert Loehning <robert.loehning@theqtcompany.com>
   Date:   Tue Sep 8 12:10:48 2015 +0200
   
       Squish: Avoid menu problems with Squish 6.0.0
       
       Change-Id: I43219d36f07e2b5636f84f891f94220148f4dd7f
       Reviewed-by: Christian Stenger <christian.stenger@theqtcompany.com>
   

2. Show this log in Creator.
3. Click on the said hex-string.
   Creator will try to open a diff for this change. Since there is no such change, showing the diff will fail. Creator will just display "Waiting for data..."
4. Click the "Reload Diff" icon.
   Creator crashes.

If the hex string does not reference a git commit, Creator should not try showing a diff for it at all. If Creator could not show a diff, trying to reload it should not crash Creator.