FBXLoader: Find a bug in the code that leads to load fail.

Description of the problem

Find a little bug in FBXLoader. See the code in the following picture:

when the “children” is undefined, the “fileName” is undefined, too.
and the below code doesn’t consider this condition and call fileName.indexOf.
It will result a error and load fail.
need add:
if (fileName !== undefined && (...))

Three.js version

• Dev
• r89
• …

Browser

• All of them
• Chrome
• Firefox
• Internet Explorer

OS

• All of them
• Windows
• macOS
• Linux
• Android
• iOS

Hardware Requirements (graphics card, VR Device, …)