Address drag drop error - Fixes #81246446

Don't accept this pull request. 

This is a proposed fix for the issue described in https://www.pivotaltracker.com/n/projects/874109/stories/81246446 where the "DragDrop registration did not succeed" message appears in unexpected places. The problem is the result of trying to call ShowMessageBox on a background thread rather than on the UI thread. Passing a Lambda to Invoke() might not be your ideal style but it resolves the issue without moving the code around too much. 

I believe we need to look at other locations where we catch exceptions on the background thread and consider apply this pattern as well