Drag & Drop: fix JavaScript error related to JX.$(undefined)

Summary:
This change fixes the following recent JavaScript error:

Error: Empty ID passed to JX.$()!

The regression was introduced here:

90f9da643d1622bcfff3a8903159aeefd1b42520

Closes T15272

Test Plan: - with this change I was able to open Conpherence Persistent Chat and Drop a file

Reviewers: O1 Blessed Committers, Dylsss, avivey

Reviewed By: O1 Blessed Committers, avivey

Subscribers: Dylsss, speck, tobiaswiese, Matthew, Cigaryno

Maniphest Tasks: T15272

Differential Revision: https://we.phorge.it/D25134