Stop Darktable from crashing when closing secondary darkroom window while window is in fullscreen (#18361)

* Consume unfullscreen event when closing secondary window

On macOS, when the secondary darkroom window is closed while it is currently full screened, there are events that are still pending that needs to be consumed. When the window is destroyed by gtk_widget_destroy before consuming all of the unfullscreen event, gtk_main() under dt_gui_gtk_run will throw exc_bad_access exception.

* Refactor

Simplify cleanup so that its all located in _second_window_delete_callback

* Let GTK destroy the window

Let MacOS handle with the fullscreen state change

Author: stnKrisna

src/views/darkroom.c

@@ -164,7 +164,7 @@ void cleanup(dt_view_t *self)
     else
       dt_conf_set_bool("second_window/last_visible", FALSE);
 
-    gtk_widget_destroy(dev->second_wnd);
+    gtk_window_close(GTK_WINDOW(dev->second_wnd)); // Use close so that _second_window_delete_callback can clean up
     dev->second_wnd = NULL;
     dev->preview2.widget = NULL;
   }
@@ -1392,7 +1392,7 @@ static void _second_window_quickbutton_clicked(GtkWidget *w, dt_develop_t *dev)
   {
     _darkroom_ui_second_window_write_config(dev->second_wnd);
 
-    gtk_widget_destroy(dev->second_wnd);
+    gtk_window_close(GTK_WINDOW(dev->second_wnd)); // Use close so that _second_window_delete_callback can clean up
     dev->second_wnd = NULL;
     dev->preview2.widget = NULL;
   }
@@ -3560,7 +3560,13 @@ static gboolean _second_window_delete_callback(GtkWidget *widget,
                                                GdkEvent *event,
                                                dt_develop_t *dev)
 {
-  _darkroom_ui_second_window_write_config(dev->second_wnd);
+  // We need to be careful when using the second window reference from dev. It could be null at this point
+  _darkroom_ui_second_window_write_config(widget);
+
+  // There's a bug in GTK+3 where fullscreen GTK window on macOS may cause EXC_BAD_ACCESS.
+  // We need to unfullscreen the window and consume all pending events first before
+  // destroying the window
+  gtk_window_unfullscreen(GTK_WINDOW(widget));
 
   dev->second_wnd = NULL;
   dev->preview2.widget = NULL;