DUI: fix tooltip in Revit.

# Solution
Ok, it was obvious... Instead `Application.Current.MainWindow` we should use `dynamoWindow`, because now MainWindow is not Dynamo, now it's Revit.
# Reviewers
@Benglin,@vmoyseenko, please take a look.

Tasks on YouTrack:
[MAGN-4970](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4970) DUI: Hovering on member crashes Dynamo