removeShadowCaster null exception

Hi,

found case that can trigger null pointer for removeShadowCaster, (possibly addShadowCaster as well).

As includeDescendants is true by default it tries to remove shadow from children as well, but _children array can be null.

if (includeDescendants) {
   // mesh.getChildren() can be null 
   for (var child of mesh.getChildren()) {
      this.removeShadowCaster(<any>child);
   }
}

So one version is to make check before:

var children = mesh.getChildren();
if (includeDescendants && children) {
   for (var child of children) {
      this.removeShadowCaster(<any>child);
   }
}

Same issue is on addShadowCaster as it tries to push children in the list