In the Commit panel, select a .suo file that has change (Visual studio user config file. a binary file.)
The diff panel shows two buttons - Open 'Before'" and "Open 'After'".

• • Assume that the .suo file extension is not associated with any program in Windows.

If you click the 'After' button, a Windows pop-up message asks you to chose a program to open the file.
If you click the 'Before' button, SourceTree crashes...

This does not happen after you associate a program to the file extension,