Make sure that we have the lower_layer_edge_grid when placing seams

src/slic3r/GUI/GUI_App.cpp

@@ -673,6 +673,7 @@ void GUI_App::post_init()
     // to popup a modal dialog on start without screwing combo boxes.
     // This is ugly but I honestly found no better way to do it.
     // Neither wxShowEvent nor wxWindowCreateEvent work reliably.
+    assert(this->preset_updater); // FIXME Following condition is probably not neccessary.
     if (this->preset_updater) {
         this->check_updates(false);
         CallAfter([this] {