Steps to reproduce:
1. Open User-supplied project
2. In Project window, create a new Sub Graph in Create->Shader->Sub Graph
3. Select the newly created graph and in the Inspector window press 'Open Shader Editor'
4. In the Shader window, press 'Add Slot' then 'Save Asset' then close the Shader window
5. Try opening the same graph again

Expected: shader node and window load in and there are no errors
Actual: the shader window fails to load in and the console throws a "NullReferenceException: Object reference not set to an instance of an object" error

Reproduced in: 2018.1.7f1, 2018.2.0f1, 2018.2.0f2, 2018.2.0f2 (macOS 10.13)

Note:
Older versions prior to 2018.1 were not tested due to Shader Graph not being implemented yet
2018.3 was not tested due to Pack-Man errors

NullReferenceException: Object reference not set to an instance of an object
UnityEditor.ShaderGraph.Drawing.Slots.MultiFloatSlotControlView..ctor (INode node, System.String[] labels, System.Func`1 get, System.Action`1 set) (at C:/Users/Student QA/AppData/Local/Unity/cache/packages/packages.unity.com/com.unity.shadergraph@1.1.3-preview/Editor/Drawing/Views/Slots/MultiFloatSlotControlView.cs:23)
UnityEditor.ShaderGraph.Vector4MaterialSlot.InstantiateControl () (at C:/Users/Student QA/AppData/Local/Unity/cache/packages/packages.unity.com/com.unity.shadergraph@1.1.3-preview/Editor/Data/Graphs/Vector4MaterialSlot.cs:52)
UnityEditor.ShaderGraph.Drawing.PortInputView..ctor (UnityEditor.ShaderGraph.MaterialSlot slot) (at C:/Users/Student QA/AppData/Local/Unity/cache/packages/packages.unity.com/com.unity.shadergraph@1.1.3-preview/Editor/Drawing/Views/PortInputView.cs:51)
UnityEditor.ShaderGraph.Drawing.MaterialNodeView.UpdatePortInputs () (at C:/Users/Student QA/AppData/Local/Unity/cache/packages/packages.unity.com/com.unity.shadergraph@1.1.3-preview/Editor/Drawing/Views/MaterialNodeView.cs:341)
UnityEditor.ShaderGraph.Drawing.MaterialNodeView.Initialize (UnityEditor.ShaderGraph.AbstractMaterialNode inNode, UnityEditor.ShaderGraph.Drawing.PreviewManager previewManager, IEdgeConnectorListener connectorListener) (at C:/Users/Student QA/AppData/Local/Unity/cache/packages/packages.unity.com/com.unity.shadergraph@1.1.3-preview/Editor/Drawing/Views/MaterialNodeView.cs:126)
UnityEditor.ShaderGraph.Drawing.GraphEditorView.AddNode (INode node) (at C:/Users/Student QA/AppData/Local/Unity/cache/packages/packages.unity.com/com.unity.shadergraph@1.1.3-preview/Editor/Drawing/Views/GraphEditorView.cs:335)
UnityEditor.ShaderGraph.Drawing.GraphEditorView..ctor (UnityEditor.EditorWindow editorWindow, UnityEditor.ShaderGraph.AbstractMaterialGraph graph, System.String assetName) (at C:/Users/Student QA/AppData/Local/Unity/cache/packages/packages.unity.com/com.unity.shadergraph@1.1.3-preview/Editor/Drawing/Views/GraphEditorView.cs:149)
UnityEditor.ShaderGraph.Drawing.MaterialGraphEditWindow.Initialize (System.String assetGuid) (at C:/Users/Student QA/AppData/Local/Unity/cache/packages/packages.unity.com/com.unity.shadergraph@1.1.3-preview/Editor/Drawing/MaterialGraphEditWindow.cs:496)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)