There is a reason the original `Trigger.timer` was marked `@CheckForNull` (by me, IIRC): some code was _not_ checking for null, and throwing NPEs under various conditions (e.g. stuff run after Jenkins shutdown, or from a unit test, etc.).  Your replacement with a `get()` that can return null reintroduces those bugs.

I would suggest getting rid of the `initialize` method, making `get` be `@Nonnull` (initialize lazily), and making `get` and `shutdown` synchronized.