The log :
80.122 Script @stdlib/stdlib/core.lua:50: real number expected got nil.
stack traceback:
stdlib/stdlib/event/event.lua:419: in function 'raise_event'
PickerBeltTools/scripts/beltbrush.lua:308: in function <PickerBeltTools/scripts/beltbrush.lua:271>
[C]: in function 'pcall'
stdlib/stdlib/event/event.lua:302: in function 'dispatch_event'
stdlib/stdlib/event/event.lua:372: in function <stdlib/stdlib/event/event.lua:336>

Can you post a mod list and steps to reproduce as I can't reproduce this.

Mod list :
https://pastebin.com/dvGaCRKz

There's no step to reproduce, it's just like the instruction
- I just have belt on the hand
- press keypad +
- Ctrl+Shift+R to make a corner.
- Ctrl +Shift+R again to mirror, then that error log appear, plus the blueprint is not mirrored.

I thought the culprit was Blueprint Extension mod, so i disabled that mod temporarily, load the game again, but it still happen. AFAIK, there's no other mod than that in my list that do blueprinting stuff.
So i'm baffled which one is the culprit.

Well, actually there is Ghost Scanner, and Recursive blueprint.
But those two only active if there's an entity in the map, and i don't use it.

Let me know which one of those mod in the list that you suspect is the culprit so i don't have to disabled one by one.

Update :
This is interesting, i disabled ALL mod EXCEPT various Picker Mod, on NEW GAME.
Turns out out the error is still happening, maybe certain combination of picker mod (or their setting) didn't like each other ?

The mod list, so you can confirm it's really only picker mod (and the Factorio Standard LIbrary dependency), in case i forgot to deactivate/activate something:
https://pastebin.com/Drqd0V3v

Ahhhhhhhh I think I see. It should work if you add the Picker Blueprinter mod. In the meantime I will see what it is going to take to make it work without that mod.

Yeah i didn't use Picker Blueprinter, i don't need the functionality that mod offer.
Also, the mantra goes "Less mod, Less conflict, Less error", turns out that mantra is wrong on this particular case, 😁

It's okay though, it doesn't crash to desktop, only threw some error.
And also there's feature from Blueprint Extension (BE) mod to flip blueprint, so instead of Ctrl+Shift+R; at the moment i just have to drop that Picker temporary blueprint to BE icon (somehow using their flipping hotkey doesn't work on this case)