I get the old NullReferenceException when I click on my WPF diagram.  Note that I can override OnPreviewMouseDown and it executes okay and, when I'm in there, my Diagram object is instantiated, as is the Content property.  Here's a bit of the stack trace:

  Source="MindFusion.Diagramming.Wpf"
  StackTrace:
      at MindFusion.Diagramming.Wpf.Behaviors.BehaviorBase.xbd9e665b67c37c5a(DiagramItem xccb63ca5f63dc470)
      at MindFusion.Diagramming.Wpf.Behaviors.LinkNodesBehavior.StartDraw(Point point)
      at MindFusion.Diagramming.Wpf.Behaviors.BehaviorBase.x4f2e1c1258f17d5e(Point x2f7096dac971d6ec, MouseButton x128517d7ded59312)
      at MindFusion.Diagramming.Wpf.Behaviors.BehaviorBase.x2e2bbfe11746fd86(Point x70c02b32ea858ee7, MouseButton x2092ce7be83a9db0)
      at MindFusion.Diagramming.Wpf.Diagram.x05e6230be73f4e2a(MouseButtonEventArgs xfbf34718e704c6bc, Point xb30c565aa19aece6)
      at MindFusion.Diagramming.Wpf.Diagram.OnPreviewMouseDown(MouseButtonEventArgs e)