Null reference exception by using IfcSurfaceTexture

[Noranius]

When applying an IfcImageTexture the Xplorer crashes with a null pointer exception. This circumstance is related to the line below because the material is not initialized before. Maybe just check for null before and if so, use a default material. https://github.com/xBimTeam/XbimWindowsUI/blob/6923e3c97cc0e374ec3a222db2d4b192063ec3a5/Xbim.Presentation/WpfMaterial.cs#L59