If you have a lot of Nulls with PSR values, and you would like to have them all replaced by an object:
Use the Main Menu> Tools> Replace With… (note the cogwheel near the menu for extra options!)
If you like to keep the Nulls, perhaps create a copy first.
There is little extra work if Nulls are the initial object: they have no Phong tag, for example. But that is typically not a big deal to fix.
Since I have no idea about your setup, please test this with a backup.
Select the Nulls
Call up the Replace with … function
Click on the object
Let me know if that works, if not please share the file and some information about what happened.
All the best