Bug 798863 - Crash on clicking Settings button second time

Ensure that dialog resources stored in options are freed when the
dialog is destroyed.

The crash happened when a new dialog replaced the old one on the options
and the old one's destructors tried to access a dangling reference to
a GtkWidget.

gnucash/gnome-utils/dialog-options.cpp

@@ -629,6 +629,13 @@ GncOptionsDialog::~GncOptionsDialog()
     gnc_unregister_gui_component_by_data(m_component_class, this);
     g_signal_handlers_disconnect_by_func(m_window, (gpointer)dialog_destroy_cb, this);
     g_signal_handlers_disconnect_by_func(m_window, (gpointer)dialog_window_key_press_cb, this);
+    m_option_db->foreach_section([](GncOptionSectionPtr& section)
+    {
+        section->foreach_option([](GncOption& option)
+        {
+            option.set_ui_item(std::unique_ptr<GncOptionUIItem>(nullptr));
+        });
+    });
     g_object_unref(m_window);
 }
 

libgnucash/engine/gnc-option.cpp

@@ -221,7 +221,8 @@ GncOption::set_ui_item(GncOptionUIItemPtr&& ui_item)
                                       return option.get_ui_type();
                                   }, *m_option);
 
-    if (ui_item->get_ui_type() != opt_ui_type)
+    //ui_item may be nullptr to free the old m_ui_item.
+    if (ui_item && ui_item->get_ui_type() != opt_ui_type)
     {
         PERR("Setting option %s:%s UI element failed, mismatched UI types.",
               get_section().c_str(), get_name().c_str());