fix bug: crash when FT_StartScene unregisterd
fix bug: crash when FT_StartScene unregisterd

MewX/fountain-game-engine,MewX/fountain-game-engine,fountainment/FountainEngine,fountainment/FountainEngine,fountainment/FountainEngine,MewX/fountain-game-engine,fountainment/FountainEngine,MewX/fountain-game-engine