shouldDeleteModernInstanceMethod shouldn’t call get()

The `shouldDeleteModernInstanceMethod` assumes that if `prototypeDescription.get` exists, that `get()` will return something. When using MobX’s `@action`, the decorator creates a `get` that returns `undefined`, which causes an exception when accessing `length` on line 31.

I’m not sure if a `get()` result of `undefined` should be treated like a length of 0 or `get` not existing in the first place. (Or if this issue really belongs to MobX.)