--- a/devtools/server/actors/chrome.js
+++ b/devtools/server/actors/chrome.js
@@ -99,18 +99,20 @@ Object.defineProperty(ChromeActor.protot
   }
 });
 
 ChromeActor.prototype.observe = function (subject, topic, data) {
   TabActor.prototype.observe.call(this, subject, topic, data);
   if (!this.attached) {
     return;
   }
+
+  subject.QueryInterface(Ci.nsIDocShell);
+
   if (topic == "chrome-webnavigation-create") {
-    subject.QueryInterface(Ci.nsIDocShell);
     this._onDocShellCreated(subject);
   } else if (topic == "chrome-webnavigation-destroy") {
     this._onDocShellDestroy(subject);
   }
 };
 
 ChromeActor.prototype._attach = function () {
   if (this.attached) {

--- a/devtools/server/actors/tab.js
+++ b/devtools/server/actors/tab.js
@@ -680,18 +680,20 @@ TabActor.prototype = {
   },
 
   observe(subject, topic, data) {
     // Ignore any event that comes before/after the tab actor is attached
     // That typically happens during firefox shutdown.
     if (!this.attached) {
       return;
     }
+
+    subject.QueryInterface(Ci.nsIDocShell);
+
     if (topic == "webnavigation-create") {
-      subject.QueryInterface(Ci.nsIDocShell);
       this._onDocShellCreated(subject);
     } else if (topic == "webnavigation-destroy") {
       this._onDocShellDestroy(subject);
     }
   },
 
   _onDocShellCreated(docShell) {
     // (chrome-)webnavigation-create is fired very early during docshell