the classroom's materials.xml is incorrect

[lishu1980]

in code for loading MaterialIoXML::LoadMaterials
find element "Inputs" and "Materials"

but in materials.xml file, no the xml node exists, in root of xml

[darksylinc]

The commit that changed how materials are parsed is 8b2b466

It previously iterated through every element. But now it expects a root "Materials" element, plus other more changes that keep causing more and more crashes.