Just discovered some long term instability in using the Crystal Report WPF viewer was down to its handling of loading reports when not visible.
If you do this
this.viewer.ViewerCore.ReportSource = reportFile;
when
this.viewer.IsVisible == false
then at some point soon after a Null reference exception is thrown in the viewer dll.
I've had to work around it by not assigning the report until I know the viewer is actually visible.