Fix #79. Do not lookup agents when destroyed.

Author: reid

lib/hub/target.js

@@ -92,6 +92,9 @@ Target.prototype.getName = function () {
  * @return {Agent} Agent object.
  */
 Target.prototype.getAgent = function (agentId) {
+    if (this.destroyed) {
+        return;
+    }
     return this.agents[agentId];
 };