:beetle: Fix game crash when quitting having balls

Game tree is null when game is exiting, causing crash on
ball_tree_exited event.

godot/component/entity/launcher/Launcher.gd

@@ -112,6 +112,9 @@ func _on_Launcher_get_firing():
 
 
 func _on_Ball_tree_exited():
+	if get_tree() == null:
+		return # Game is exiting
+
 	load_bullet(1)
 	if (get_tree().get_nodes_in_group("ball").size() == 0):
 		emit_signal("is_full")