Fix https://forum.babylonjs.com/t/assetcontainer-dispose-does-not-work/2971
Fix https://forum.babylonjs.com/t/assetcontainer-dispose-does-not-work/2971

BabylonJS/Babylon.js,sebavan/Babylon.js,Kesshi/Babylon.js,sebavan/Babylon.js,Kesshi/Babylon.js,RaananW/Babylon.js,NicolasBuecher/Babylon.js,BabylonJS/Babylon.js,NicolasBuecher/Babylon.js,jbousquie/Babylon.js,sebavan/Babylon.js,RaananW/Babylon.js,Kesshi/Babylon.js,NicolasBuecher/Babylon.js,RaananW/Babylon.js,jbousquie/Babylon.js,BabylonJS/Babylon.js,jbousquie/Babylon.js