Reproduction instructions:
- Define subtype `X`.
- Create a bunch of tasks with subtype `X`.
- Remove the definition of subtype `X`.
Anything which views one of these objects (e.g., the home page, the task list) will fatal.
It would probably be better to let the object //act// as though it was of the default subtype, and provide some hints in the UI that the subtype is unknown.
A cheaper approach is to filter these objects out in the Query layer. This is "free", but makes all the objects of subtype "X" vanish mysteriously when you undefine subtype "X".