Fix

Examples/SimpleGraph/MainWindow.xaml.cs

@@ -85,7 +85,7 @@ namespace SimpleGraph
 
             //This method sets edges arrows visibility. It is also applied to all edges in Area.EdgesList. You can also set property for
             //each edge individually using property, for ex: Area.EdgesList[0].ShowArrows = true;
-            Area.ShowAllEdgesArrows(true);
+            Area.ShowAllEdgesArrows(false);
 
             //This method sets edges labels visibility. It is also applied to all edges in Area.EdgesList. You can also set property for
             //each edge individually using property, for ex: Area.EdgesList[0].ShowLabel = true;

GraphX.PCL.Logic/Algorithms/LayoutAlgorithms/Tree/SimpleTreeLayoutAlgorithm.cs

@@ -24,7 +24,7 @@ namespace GraphX.PCL.Logic.Algorithms.LayoutAlgorithms
         public SimpleTreeLayoutAlgorithm( TGraph visitedGraph, IDictionary<TVertex, Point> vertexPositions, IDictionary<TVertex, Size> vertexSizes, SimpleTreeLayoutParameters parameters )
             : base( visitedGraph, vertexPositions, parameters )
         {
-            VertexSizes = new Dictionary<TVertex, Size>(vertexSizes);
+            VertexSizes = VertexSizes == null ? new Dictionary<TVertex, Size>() : new Dictionary<TVertex, Size>(vertexSizes);
         }
 
         public override void Compute(CancellationToken cancellationToken)