remove undefined; only use null now

scripts/engine.cm

@@ -375,10 +375,10 @@ $_.clock = function(fn) {
 $_.clock[cell.DOC] = "takes a function input value that will eventually be called with the current time in number form."
 
 var underlings = new Set() // this is more like "all actors that are notified when we die"
-var overling = undefined
-var root = undefined
+var overling = null
+var root = null
 
-var receive_fn = undefined
+var receive_fn = null
 var greeters = {} // Router functions for when messages are received for a specific actor
 
 globalThis.is_actor = function is_actor(actor) {
@@ -426,8 +426,8 @@ $_.connection[cell.DOC] = "The connection function takes a callback function, an
 var peers = {}
 var id_address = {}
 var peer_queue = new Map()
-var portal = undefined
-var portal_fn = undefined
+var portal = null
+var portal_fn = null
 
 $_.portal = function(fn, port) {
   if (portal) throw new Error(`Already started a portal listening on ${portal.port}`)
@@ -649,7 +649,7 @@ globalThis.send = function send(actor, message, reply) {
     replies[id] = reply
     $_.delay(_ => {
       if (replies[id]) {
-        replies[id](undefined, "timeout")
+        replies[id](null, "timeout")
         delete replies[id]
       }
     }, REPLYTIMEOUT)