You can work around it by editing your preferences file to stick a value in that entry.
1) Open prefs/boot.txt
2) Edit the line starting with "build" and give it the value: 404
3) Check the line that starts with "version" and call it 1.0.4. It really doesn't matter what this one is. It's entirely visual.
(If those lines don't exist, create them)
4) Save and close
Why it's turning null to begin with, I have no idea how that could happen. I could have it auto-correct the version if it's null by dunking in the version of the latest build.