Handle case when element is undefined

[arpowers]

This PR implements or fixes...

This prevents an error in a case when dragging elements to an empty container. In those cases, the element ends up being undefined throwing an error. I thought the easiest way to fix would be just using optional chaining.

Does this PR require the Docs to be updated?

No

Does this PR require new tests?

No

This branch been tested on... (click all that apply / add new items)

Browsers:

• [ x] Chrome version
• [ ] Firefox version
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• [ ] IE / Edge version
• [ ] iOS Browser version
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