restore compatibility for tag 'MaterialModel'

GUI/Model/Sample/SampleItem.cpp

@@ -200,7 +200,7 @@ void SampleItem::readFrom(QXmlStreamReader* r)
             XML::gotoEndElementOfTag(r, tag);
 
             // materials
-        } else if (tag == Tag::MaterialsSet) {
+        } else if (tag == Tag::MaterialsSet || tag == "MaterialModel" /*v<=21*/) {
             m_materials.readFrom(r);
             XML::gotoEndElementOfTag(r, tag);