Bug 1014015 - Fix webapps actor exception when connecting more than once. r=paul

toolkit/devtools/server/actors/webapps.js

@@ -204,6 +204,10 @@ WebappsActor.prototype = {
   actorPrefix: "webapps",
 
   disconnect: function () {
+    try {
+      this.unwatchApps();
+    } catch(e) {}
+
     // When we stop using this actor, we should ensure removing all files.
     for (let upload of this._uploads) {
       upload.remove();