[BUGFIX] Use function assignment to keep binding of `this`

Define the `hideDropzone` event listener via assignment to keep the
binding of `this` to the DragUploader class.

Resolves: #90792
Releases: master, 9.5
Change-Id: I21c52da477349e77a52f2c6701fd75abc3b5b074
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/63805


Tested-by: Susanne Moog <look@susi.dev>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Susanne Moog <look@susi.dev>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>

Build/Sources/TypeScript/backend/Resources/Public/TypeScript/DragUploader.ts

@@ -202,7 +202,7 @@ class DragUploaderPlugin {
    *
    * @param {Event} event
    */
-  public hideDropzone(event: Event): void {
+  public hideDropzone = (event: Event): void => {
     event.stopPropagation();
     event.preventDefault();
     this.$dropzone.hide();