Actually this is pretty obvious because the marker looks for a box to initialize (for example) and doesn't find one because the blueprint doesn't contain any data. I hope I got that right...

I made a very simple fix that allows placing a blank blueprint on the marker without anything happening. If this is good enough then good, but if not, then I need a day to figure out how to blacklist blank blueprints because now it's 2:40 AM where I live and I can't think right now :p

I'll also add that this problem causes the world to be unplayable because the marker ticks and crashes right after the world loads

Will take this for 6.0 - can you share a log?

This is actually for 6.1 because the construction marker is not in 6.0, and sure, I'll paste the crash log tomorrow (if I didn't delete it :p (no worries I can reproduce if I need to)).