focus stack and next/prev toplevel keybinds

src/main.c

@@ -89,6 +89,8 @@ int main(int argc, char **argv)
     server.scene_layout = wlr_scene_attach_output_layout(server.scene, server.output_layout);
 
     wl_list_init(&server.toplevels);
+    wl_list_init(&server.focus_stack);
+
     server.xdg_shell = wlr_xdg_shell_create(server.display, 3);
     server.new_xdg_toplevel.notify = server_new_xdg_toplevel;
     wl_signal_add(&server.xdg_shell->events.new_toplevel, &server.new_xdg_toplevel);