fix (addon): #1013 only send tab data if there is an active tab (#1031)

Fixes #1013 
Closes #1031

lib/TabTracker.js

@@ -115,6 +115,11 @@ TabTracker.prototype = {
   },
 
   handlePerformanceEvent(eventData, eventName, value) {
+    if (!tabs.activeTab) {
+      // short circuit out if there is no active tab
+      return;
+    }
+
     let payload = Object.assign({}, eventData);
     payload.action = "activity_stream_performance";
     payload.tab_id = tabs.activeTab.id;