diff -Nru gnome-shell-extension-ubuntu-dock-63ubuntu1.18.10.1/debian/changelog gnome-shell-extension-ubuntu-dock-63ubuntu1.18.10.2/debian/changelog
--- gnome-shell-extension-ubuntu-dock-63ubuntu1.18.10.1/debian/changelog	2018-12-19 09:07:53.000000000 +0000
+++ gnome-shell-extension-ubuntu-dock-63ubuntu1.18.10.2/debian/changelog	2019-01-15 21:35:34.000000000 +0000
@@ -1,3 +1,10 @@
+gnome-shell-extension-ubuntu-dock (63ubuntu1.18.10.2) cosmic; urgency=medium
+
+  [ Andrea Azzarone ]
+  * theming: Ensure _trackingWindows contains valid windows (LP: #1769383)
+
+ -- Marco Trevisan (Treviño) <marco@ubuntu.com>  Tue, 15 Jan 2019 22:35:34 +0100
+
 gnome-shell-extension-ubuntu-dock (63ubuntu1.18.10.1) cosmic; urgency=medium
 
   [ Marco Trevisan (Treviño) ]
diff -Nru gnome-shell-extension-ubuntu-dock-63ubuntu1.18.10.1/theming.js gnome-shell-extension-ubuntu-dock-63ubuntu1.18.10.2/theming.js
--- gnome-shell-extension-ubuntu-dock-63ubuntu1.18.10.1/theming.js	2018-08-02 11:29:47.000000000 +0000
+++ gnome-shell-extension-ubuntu-dock-63ubuntu1.18.10.2/theming.js	2019-01-15 21:35:34.000000000 +0000
@@ -388,11 +388,13 @@
         ]);
 
         // Window signals
-        global.get_window_actors().forEach(function(win) {
+        global.window_group.get_children().filter(function(child) {
             // An irrelevant window actor ('Gnome-shell') produces an error when the signals are
             // disconnected, therefore do not add signals to it.
-            if (win.get_meta_window().get_wm_class() !== 'Gnome-shell')
-                this._onWindowActorAdded(null, win);
+            return child instanceof Meta.WindowActor &&
+                   child.get_meta_window().get_wm_class() !== 'Gnome-shell';
+        }).forEach(function(win) {
+            this._onWindowActorAdded(null, win);
         }, this);
 
         if (this._settings.get_enum('transparency-mode') === TransparencyMode.ADAPTIVE)