message: Fix rare crash when default material is changed from OBJ
  This fixes a rare crash where the default material (sgDefault) has
  been changed to a type not handled by the current renderer (scivis or AO).
diff: @@ -56,6 +56,14 @@ void MaterialRegistry::preCommit()
  if (rType == "debug")
  return;
  + // If the default material (sgDefault) has been changed to a type not handled
  + // by the current renderer, recreate it as obj (the universal material type).
  + // XXX this too will be fixed by the generalized materials.
  + if (!child("sgDefault")["handles"].hasChild(rType)) {
  + createChild("sgDefault", "obj");
  + traverse<sg::GenerateOSPRayMaterials>(rType);
  + }
  +
  auto defaultMaterial =
  child("sgDefault")["handles"].child(rType).nodeAs<sg::Material>();
  auto &defaultCppMaterial = defaultMaterial->valueAs<cpp::Material>();
  