问导出失败的AugmentedFacesExample在iOS 14和EXC_BAD_ACCESS上失败

EN

    // Added
    let mdlMesh = MDLMesh(scnGeometry: faceTextureNode.geometry!)
    let asset = MDLAsset()
    asset.add(mdlMesh)
    do {
        let directory = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first
        let obj_url = directory!.appendingPathComponent("test_\(id).obj")
        try asset.export(to: obj_url) // iOS14
        self.id += 1
    }
    catch {
        print("SCNSceneRendererDelegate:renderer :: Can't write mesh to url")
    }