Nodes Disappearing when deleting, freezes unity

I have a large graph, it seems when I delete a node (or break a connection) sometimes it causes unity to freeze, and some node boxes disappear.  This happens inconsistently.  

I'm required to force quit unity and reopen, then I can delete the node without problems, but it happens on another node causes a freeze again.

Throws this error: NullReferenceException: Object reference not set to an instance of an object
ShaderForge.SF_Node.GetInputData (System.String id)
ShaderForge.SFN_Lerp.IsUniformOutput ()
ShaderForge.SFN_If.IsUniformOutput ()
ShaderForge.SF_Node.Draw ()
ShaderForge.SF_EditorNodeView.OnLocalGUI (Rect r)
ShaderForge.SF_Editor.OnGUI ()
System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at /Users/builduser/buildslave/mono-runtime-and-classlibs/build/mcs/class/corlib/System.Reflection/MonoMethod.cs:222)