Possible fix for multi-thread usage of DialogHost.GetInstance() (#2849)

* Possible fix for multi-thread usage of DialogHost.GetInstance()

* Modify test cleanup

Replace obsolete Thread.Abort() with proper shutdown of Dispatcher instead.

* Whitespace commit to fore re-run

* Cleaning up the setting of the identifier

Co-authored-by: Kevin Bost <[email protected]>

Author: nicolaihenriksen