[BUG] Prevent setvar to null value

[mercfate-sys]

Using ClassicUO

How to prevent a variable to be NULL?

i have this 2 scripts:

set-shield
setvar 'shield'

equip-shield
dress 'shield'

When i use setvar script, a target opens to select the shield but if a press ESC key, Razor bugs and not working anymore then i need to close UO and Razor.
Is it possible to check if a variable is NULL before Razor assign it?

Thanks in advance.
Razor CE version: 1.9.77.0