Fix receiving payloads using a bluetooth connection

Summary:
This fixes receiving payloads using the bluetooth backend.

This looks like some bug in Qt, but I couldn't figure out what happens exactly (will report a bug though).

Test Plan: Receiving files now works!

Reviewers: KDE Connect, apol

Reviewed By: KDE Connect, apol

Subscribers: kdeconnect, nicolasfella, apol, KDE Connect

Tags: KDE Connect

Differential Revision: https://phabricator.kde.org/D12153