NullReferenceException in ConstrainedDelaunayTriangulation.CdtSweeper

referencing nuget AutomaticGraphLayout.GraphViewerGDI 1.1.12

Graph graph = new Graph("Test")
{
    LayoutAlgorithmSettings = new MdsLayoutSettings()
};
graph.AddEdge("B", "C");
graph.AddEdge("A", "D");
graph.AddEdge("A", "B");

var viewer = new Microsoft.Msagl.GraphViewerGdi.GViewer();
viewer.Graph = graph;

Microsoft.Msagl.Routing.ConstrainedDelaunayTriangulation.CdtSweeper.CreatePerimeterElementFromEdge(Microsoft.Msagl.Routing.CdtEdge)
Microsoft.Msagl.Routing.ConstrainedDelaunayTriangulation.CdtSweeper.CreateDoubleLinkedListOfPerimeter()
Microsoft.Msagl.Routing.ConstrainedDelaunayTriangulation.CdtSweeper.FinalizeTriangulation()
Microsoft.Msagl.Core.AlgorithmBase.Run()
Microsoft.Msagl.Core.AlgorithmBase.Run()
Microsoft.Msagl.Core.Layout.ProximityOverlapRemoval.MinimumSpanningTree.GTreeOverlapRemoval.OneIteration(Microsoft.Msagl.Core.Geometry.Point[], Microsoft.Msagl.Core.DataStructures.Size[], bool)
Microsoft.Msagl.Core.Layout.ProximityOverlapRemoval.MinimumSpanningTree.GTreeOverlapRemoval.RemoveOverlaps()
Microsoft.Msagl.Layout.MDS.MdsGraphLayout.LayoutConnectedGraphWithMds(Microsoft.Msagl.Core.Layout.GeometryGraph)
System.Threading.Tasks.Parallel.ForWorker.AnonymousMethod__1()
System.Threading.Tasks.Task.InnerInvokeWithArg(System.Threading.Tasks.Task)

Reproduce the report using the Graph, MdsLayoutSettings, and GViewer example, then start at ConstrainedDelaunayTriangulation.CdtSweeper.CreatePerimeterElementFromEdge and follow CreateDoubleLinkedListOfPerimeter and FinalizeTriangulation. Use the stack trace to inspect the failing triangulation path; done means the sample graph completes layout without a NullReferenceException.