question

From the user manual:

In general, to check if a treenode is null just compare the treenode with NULL or put it directly in an if statement. Note that the keyword nullvar represents a null Variant and is not the same as a null treenode.

treenode fred = model().find("doesnotexist");
return fred == NULL; // true
return fred == nullvar; // false
return tonum(fred); // 0
if (fred) return 1; else return 0; // 0
if (objectexists(fred)) return 1; else return 0; // 0

You should read the rest of the section under Null Values on this user manual page https://docs.flexsim.com/en/19.2/Reference/CodingInFlexSim/FlexScriptAPIReference/Tree/treenode.html..