HideTracker: Guard against accessing invalid pointer

src/ShellClients/HideTracker.vala

@@ -14,7 +14,7 @@ public class Gala.HideTracker : Object {
     public signal void show ();
 
     public Meta.Display display { get; construct; }
-    public unowned PanelWindow panel { get; construct; }
+    public unowned PanelWindow? panel { get; construct; }
 
     private static GLib.Settings behavior_settings;
 
@@ -37,6 +37,10 @@ public class Gala.HideTracker : Object {
 
     construct {
         panel.window.unmanaging.connect_after (() => {
+            /* The window was unmanaged so the panel will be freed because we only hold a weak reference,
+               therefore set it to null to avoid a dangling pointer */
+            panel = null;
+
             // The timeouts hold refs on us so we stay connected to signal handlers that might
             // access the panel which was already freed. To prevent that make sure we reset
             // the timeouts so that we get freed immediately
@@ -49,6 +53,10 @@ public class Gala.HideTracker : Object {
         unowned var cursor_tracker = display.get_cursor_tracker ();
 #endif
         cursor_tracker.position_invalidated.connect (() => {
+            if (panel == null) {
+                return;
+            }
+
             var has_pointer = panel.window.has_pointer ();
 
             if (hovered != has_pointer) {
@@ -72,6 +80,10 @@ public class Gala.HideTracker : Object {
 
     private void on_window_created (Meta.Window new_window) {
         InternalUtils.wait_for_window_actor (new_window, (new_window_actor) => {
+            if (panel == null) {
+                return;
+            }
+
             if (!panel.window.is_ancestor_of_transient (new_window_actor.meta_window)) {
                 return;
             }
@@ -87,6 +99,10 @@ public class Gala.HideTracker : Object {
     }
 
     private void check_trigger_conditions () {
+        if (panel == null) {
+            return;
+        }
+
         if (hovered || has_transients || panel.window.has_focus ()) {
             trigger_show ();
         } else {
@@ -119,6 +135,10 @@ public class Gala.HideTracker : Object {
     }
 
     private void setup_barrier () {
+        if (panel == null) {
+            return;
+        }
+
         var monitor_geom = display.get_monitor_geometry (display.get_primary_monitor ());
         var scale = Utils.get_ui_scaling_factor (display, display.get_primary_monitor ());
         var offset = Utils.scale_to_int (BARRIER_OFFSET, scale);
@@ -174,7 +194,7 @@ public class Gala.HideTracker : Object {
 
     private void on_barrier_triggered () {
         // Showing panels in fullscreen is broken in X11
-        if (InternalUtils.get_x11_in_fullscreen (display)) {
+        if (InternalUtils.get_x11_in_fullscreen (display) || panel == null) {
             return;
         }