Fix critical when right-clicking on the document notebook outside a tab

src/notebook.c

@@ -503,7 +503,7 @@ static void show_tab_bar_popup_menu(GdkEventButton *event, GeanyDocument *doc)
 	gtk_widget_show(menu_item);
 	gtk_container_add(GTK_CONTAINER(menu), menu_item);
 	g_signal_connect(menu_item, "activate", G_CALLBACK(on_close_documents_right_activate), doc);
-	gtk_widget_set_sensitive(GTK_WIDGET(menu_item), has_tabs_on_right(doc));
+	gtk_widget_set_sensitive(GTK_WIDGET(menu_item), doc != NULL && has_tabs_on_right(doc));
 
 	menu_item = ui_image_menu_item_new(GTK_STOCK_CLOSE, _("C_lose All"));
 	gtk_widget_show(menu_item);