fix: check if callback is null before invoking

Author: f2prateek

lib/index.js

@@ -440,9 +440,12 @@ Segment.prototype.retrieveCrossDomainId = function(callback) {
 
   var cachedCrossDomainId = this.getCachedCrossDomainId();
   if (cachedCrossDomainId) {
-    callback(null, {
-      crossDomainId: cachedCrossDomainId
-    });
+    // Callback is only provided in tests.
+    if (callback) {
+      callback(null, {
+        crossDomainId: cachedCrossDomainId
+      });
+    }
     return;
   }
 
@@ -461,11 +464,12 @@ Segment.prototype.retrieveCrossDomainId = function(callback) {
 
   getCrossDomainIdFromServerList(domains, writeKey, function(err, res) {
     if (err) {
-      // We optimize for no conflicting xid as much as possible. So bail out if there is an
-      // error and we cannot be sure that xid does not exist on any other domains
+      // Callback is only provided in tests.
       if (callback) {
         callback(err, null);
       }
+      // We optimize for no conflicting xid as much as possible. So bail out if there is an
+      // error and we cannot be sure that xid does not exist on any other domains.
       return;
     }