A customer mentioned to me the other day that the PPG didn’t pop up when he created a null.

It turns out we never pop-up the PPG when you create a null. I just never noticed that.
To verify that, I checked the implementation of GetPrim, which is the script GetPrimProc in %XSI_HOME%\Application\DSScripts\primitives.vbs.

In GetPrimProc, you’ll see that we skip nulls when we call AutoInspect:

'--------------------------------------------------------------------
' Inspect the new object
'--------------------------------------------------------------------
if Not TypeName (out_primitive) = "Nothing" then
	if  Not out_primitive.type = "null" then
		AutoInspect GetPrimProc, ,, "General"
	end if
end if

The customer also had another problem: pressing ENTER didn’t open up the PPG either, or if it did, all he got was a numeric slider for setting the Icon. Restarting Softimage fixed this; my guess is that the cached PPG layout was corrupted somehow (Softimage caches PPG layout definitions–if you’ve spent any time coding custom properties you’ve probably noticed how the PPGs of existing properties don’t change when you change the layout code).