Make sure the active window is not destroyed.

lib/screenshooter-capture.c

@@ -86,6 +86,15 @@ static GdkWindow
     {
       TRACE ("No active window, fallback to the root window");
 
+      window = gdk_get_default_root_window ();
+      *needs_unref = FALSE;
+      *border = FALSE;
+    }
+  else if (G_UNLIKELY (gdk_window_is_destroyed (window)))
+    {
+      TRACE ("The active window is destroyed, fallback to the root window.");
+
+      g_object_unref (window);
       window = gdk_get_default_root_window ();
       *needs_unref = FALSE;
       *border = FALSE;