For other newbies to .Net Runtime working thru the tutorials.
Tutorial Author, edit, and save maps to your portal—ArcGIS Runtime SDK for .NET | ArcGIS for Developers
1) At the end of the XAML GRID block that lays out the text boxes:
change <esri:MapView Map="{Binding Map, Source={StaticResource MapViewModel}}" Grid.Column="1"/>
to
<esri:MapView x:Name="MyMapView" Map="{Binding Map, Source={StaticResource MapViewModel}}" Grid.Column="1"/>
2) For method "OnSaveMapClick" in MainWindow.xaml.cs the 'catch' section is buggered. Should be:
MessageBox.Show("Error while saving: " + ex.Message);
DG