I couldn’t spot that „Walking.material“ in the example. I guess you do it wrong, but hard to judge if we can not see the code which throws this exception…
Or I missed it?!
EDIT: Ok I saw it, so you load another model with a material or the like. Still don’t get it, better post your broken code here. And please please please use code blocks there is a sticky post on top which explains it => How to type code blocks