getting bogged down with the selectedObject

[RM]

So; here I am mucking around with my alternative front-end and have a 
merry little button
with this script in it:

on mouseUp
    put fld "nname" into NN
    set the name of the selectedObject to NN
end mouseUp

which, unsurprisingly perhaps, changes the name of a selectedObject
in the topStack [which is what it is meant to do].

However, I cannot work out how to stop the button "throwing a bluey"
when nothing is selected.

Richmond.