Everytime i do the following steps SourceTree crashes

1) Resolve the conflict by launching external tool. In my case i am using perforce merge
2) Make changes, click save and close the diff file.
3) SourceTree crashes.

The same happens when i use KDiff3 as my external diff/merge tool

System.NullReferenceException was unhandled
Message: An unhandled exception of type 'System.NullReferenceException' occurred in SourceTree.Api.UI.Wpf.dll
Additional information: Object reference not set to an instance of an object.

I tried downgrading to lower versions (1.6 and 1.7) and now it crashes on launch.