Migrating a Material Instance pulls along its preview mesh and all associated materials and textures

A licensee has complained that if you assign a preview mesh to a Material Instance, it becomes a referenced asset, so migrating that MatInst will also migrate that mesh and whatever material is assigned to it. In some cases, this can bring over a bunch of stuff you don't need.
The normal material editor allows you to "clear" the preview mesh, but there seems to be no way to do that in the Material Instance editor.

*As a workaround, you can replace your preview mesh with something from engine content (cube, etc) and it won't try to migrate that.

1. Create a material and an instance of it
2. Assign a preview mesh (I used SM_Rock in Starter Content)
3. Try to migrate the Material Instance

Result: SM_Rock, its materials and textures will all be included in the migration.