[gimp] app: do not update a finalized dialog.

[Jehan <jehanp src gnome org>]

commit cc9a15de979f9fba2a4cc37b4eac1de36b888f52
Author: Jehan <jehan girinstud io>
Date:   Tue Feb 11 21:40:28 2020 +0100

    app: do not update a finalized dialog.
    
    When the about dialog is destroyed, the weak link will be nullified.
    Don't do anything then in our handler.

 app/dialogs/about-dialog.c | 3 +++
 1 file changed, 3 insertions(+)
---
diff --git a/app/dialogs/about-dialog.c b/app/dialogs/about-dialog.c
index 2b17741221..d767d830f0 100644
--- a/app/dialogs/about-dialog.c
+++ b/app/dialogs/about-dialog.c
@@ -689,6 +689,9 @@ about_dialog_last_release_changed (GimpCoreConfig   *config,
   g_signal_handlers_disconnect_by_func (config,
                                         (GCallback) about_dialog_last_release_changed,
                                         dialog);
+  if (! dialog->dialog)
+    return;
+
   if (dialog->update_frame)
     {
       gtk_widget_destroy (dialog->update_frame);