adw: clear stored tab overview timer

This prevents a GTK warning:

```
(process:354789): GLib-CRITICAL **: 19:07:01.853: Source ID 511 was not found when attempting to remove it
```

which happens when trying to clean a timer already used.

src/apprt/gtk/Window.zig

@@ -571,6 +571,7 @@ fn adwTabOverviewOpen(
 fn adwTabOverviewFocusTimer(
     self: *Window,
 ) callconv(.C) c.gboolean {
+    self.adw_tab_overview_focus_timer = null;
     self.focusCurrentTab();
 
     // Remove the timer