How do I export GameObject (including child sub-object) to .obj file?

To export a gameobject to .obj file, I'm using Obj exporter. I can do that in editor, but I'd like to do in game. I put GameObject(this includes two Cubes) in a scene, though no components in the GameObject. Once I try export it using MeshToFile, NullReferenceException comes up. I know the GameObject doesn't have meshfilter, but what I'd like to do is export whole GameObject including child sub-object. I could do in editor by selecting the GameObject and exporting.

Why can't do the same thing in game?

Could anyone help me?