Use smart pointer for the dropped node
Summary: This is necessary so that the dropped node will not be removed before it is attached to another parent node.
Test Plan: manual
Reviewers: kislinsk, O3 MITK Reviewer Group II
Reviewed By: kislinsk, O3 MITK Reviewer Group II
Subscribers: kalali, kislinsk
Maniphest Tasks: T27559
Differential Revision: https://phabricator.mitk.org/D352