The issue is that xmodules and xblocks is a pluggable architecture where we cannot guarantee any particular Xblock type exists on a server. How we handle that for courses which need those xblocks is a good question, but in this case, I'm merely filtering out non-existent types from the authoring choices. This condition would also apply if the course owner typed a non-existent type into the advanced_components list in the advanced editor; however, we shouldn't plague the perhaps less-savvy subsequent author w/ errors about missing types.