Bug 837208 - Closing a private browsing window yields NS_ERROR_FAILURE in removeObserver() at LightweightThemeConsumer.jsm:64; r=ehsan

toolkit/content/LightweightThemeConsumer.jsm

@@ -59,11 +59,14 @@ LightweightThemeConsumer.prototype = {
   },
 
   destroy: function () {
-    Components.classes["@mozilla.org/observer-service;1"]
-              .getService(Components.interfaces.nsIObserverService)
-              .removeObserver(this, "lightweight-theme-styling-update");
+    if (!PrivateBrowsingUtils.isWindowPrivate(this._win)) {
+      Components.classes["@mozilla.org/observer-service;1"]
+                .getService(Components.interfaces.nsIObserverService)
+                .removeObserver(this, "lightweight-theme-styling-update");
+
+      this._win.removeEventListener("resize", this);
+    }
 
-    this._win.removeEventListener("resize", this);
     this._win = this._doc = null;
   },