ember-sortable: [2.2.0-beta.0] Modifiers: Breaks when items are loaded asynchronously

When using the new modifiers syntax, it seems to break with several errors being thrown if the array of items being sorted was loaded asynchronously.

I came across this bug when implementing a component that fetches its data outside of the normal route lifecycle.

After a small amount of debugging, I found that due to the data being fetched asynchronously, this line was returning an empty NodeList.

Since the sortable-group modifier is unaware of the items being used to render the sortable-item elements, it isn’t able to recognise that it needs to re-initialize itself.

Not really sure on the best approach for resolving this, any help would be appreciated 🙂

To Reproduce Steps to reproduce the behavior:

1. Create a component that fetches data asynchronously
2. Render the items using code similar to the below
3. Attempt to reorder list of items
4. Observe errors in console (errors attached at bottom of issue)

<ol {{sortable-group onChange=this.reorderItems}}>
  {{#each this.asyncFetchedItems as |item|}}
    <li {{sortable-item model=item}}>{{item.name}}</li>
  {{/each}}
</ol>

Expected behavior {{sortable-group}} modifier should be able to handle {{sortable-item}} elements being rendered asynchronously.

Additional context

Ember: v3.15.0
EmberCLI: v3.15.1
Node: v12.14.0

I’ve attached the errors that were thrown below,

sortable-item.js:370 Uncaught TypeError: Cannot read property 'prepare' of undefined
    at SortableItemModifier._startDrag (sortable-item.js:370)
    at SortableItemModifier._prepareDrag (sortable-item.js:335)
    at Backburner._run (backburner.js:1013)
    at Backburner._join (backburner.js:989)
    at Backburner.join (backburner.js:760)
    at join (index.js:179)
    at index.js:274

backburner.js:389 Uncaught Error: You attempted to schedule an action in a queue (actions) for a method that doesn't exist
    at DeferredActionQueues.schedule (backburner.js:389)
    at Backburner._scheduleExpiredTimers (backburner.js:1122)
    at Backburner._runExpiredTimers (backburner.js:1095)

sortable-item.js:595 Uncaught TypeError: Cannot read property 'update' of undefined
    at SortableItemModifier._drop (sortable-item.js:595)
    at sortable-item.js:362
    at Backburner._run (backburner.js:1013)
    at Backburner.run (backburner.js:754)
    at run (index.js:125)
    at drop (sortable-item.js:361)

backburner.js:389 Uncaught Error: You attempted to schedule an action in a queue (actions) for a method that doesn't exist
    at DeferredActionQueues.schedule (backburner.js:389)
    at Backburner._scheduleExpiredTimers (backburner.js:1122)
    at Backburner._runExpiredTimers (backburner.js:1095)