Smarty

test1:
{assign var='varname' value='nonexisting'}
{$varname}
is
{if isset(${$varname})}
set
{else}
not set
{/if}
output:
nonexisting is
( ! ) Notice: Undefined index: nonexisting in ...
...

Why are you using such convoluted constructs to begin with?

Sadly it's the way I get the data. It's an old shop system, using multiple templates, and I can not change the basic data structure wit ...

It seems to use default like

{if ${$varname}|default:""}

is a usable workaround...

I get notices when using isset!

Simple example:

{assign var="varname" value="name_of_non_existing_variable"}
{$varname} is {if isset(${$varname})} set {else} not set {/if}
...