Thank you for your project!

QDirStat tries to read /proc/mount and /etc/mtab files to fetch mount points. All mentioned files are Linux-specific. As results, "Cross Filesystems" option causes crash on BSD systems.

On FreeBSD there is "/sbin/mount" routine to fetch mount points in same format as fstab:

/sbin/mount -p

The output of this command is sufficient. I'm not QT/C++ developer and can't provide patch to solve issue.
Dirty hack is to read pre-populated file by mount command and it works fine for me.

Please feel free to ask questions.