Fix exception when passing null as PARAM arg

lib/session.ts

@@ -113,7 +113,7 @@ export function configContextCallback(
   exp: Expression
 ): Expression {
   if (!Array.isArray(exp)) return exp;
-  if (exp[0] === "PARAM") {
+  if (exp[0] === "PARAM" && typeof exp[1] === "string") {
     let name = exp[1];
     if (name === "id") name = "DeviceID.ID";
     else if (name === "serialNumber") name = "DeviceID.SerialNumber";