this.sortableGroup.prepare() throws an error sometimes

**Error:**

> Cannot read properties of undefined (reading 'prepare')

**Steps to reproduce?**

I unfortunately don't know how to reproduce this. I have 2 Sentry Replays where I see users get this error but they don't do anything special.

For our traffic it happens **very rarely** but still happened to 30 users 🤷‍♂️

It looks like it's failing during some re-rendering & dragging but I tried 20x CPU slowdown and still no joi with reproduction.

I see some code is defensive when accessing `this.sortableGroup`:

https://github.com/adopted-ember-addons/ember-sortable/blob/5a2bba926daf36c0acaee0691c65e5b429ba1aaa/addon/src/modifiers/sortable-item.ts#L124-L130

but not in all places.

Maybe I could add a bit of defensiveness to `this.sortableGroup.prepare()` to prevent this error from being thrown?

----

Instead of simple optional chaining [I'd rather propose](https://github.com/adopted-ember-addons/ember-sortable/pull/655) this:

```diff
DRAG_ACTIONS.forEach((event) => window.addEventListener(event, dragThrottled));
END_ACTIONS.forEach((event) => window.addEventListener(event, drop));

+ if (!this.sortableGroup) return;

this.sortableGroup.prepare();
set(this, 'isDragging', true);
```

[I don't think anything below](https://github.com/adopted-ember-addons/ember-sortable/blob/5a2bba926daf36c0acaee0691c65e5b429ba1aaa/addon/src/modifiers/sortable-item.ts#L511-L514) `this.sortableGroup.prepare()` makes sense without `this.sortableGroup` being present 🤷‍♂️