Reproduction Steps
- start new project in 18.2.0b7
- update to latest LW RP (1.1.10)
- update to latest shader graph (1.1.19)
- Assets->Import Package->Custom Package to pull in the attached repro package
- Select Assets\ExampleAssets\Materials\ShaderGraph\LanternLight.ShaderGraph. In the Inspector panel click "Open Shader Editor"
- Add the CosAbs subgraph to this shadergraph. Hook up the time input from the time block already in the graph
- Wait for a minute or two

Expected:
No errors

Observed:
After some time, 2 errors will occur

(1)
Not implemented yet
UnityEditor.EditorApplication:Internal_CallUpdateFunctions()

(2)
NullReferenceException: Object reference not set to an instance of an object
UnityEditor.ShaderGraph.SubGraphNode.get_referencedGraph () (at C:/Users/jackpr/AppData/Local/Unity/cache/packages/packages.unity.com/com.unity.shadergraph@1.1.9-preview/Editor/Data/Nodes/Utility/SubGraphNode.cs:41)
UnityEditor.ShaderGraph.SubGraphNode.get_previewMode () (at C:/Users/jackpr/AppData/Local/Unity/cache/packages/packages.unity.com/com.unity.shadergraph@1.1.9-preview/Editor/Data/Nodes/Utility/SubGraphNode.cs:94)
UnityEditor.ShaderGraph.Drawing.PreviewManager.RenderPreviews () (at C:/Users/jackpr/AppData/Local/Unity/cache/packages/packages.unity.com/com.unity.shadergraph@1.1.9-preview/Editor/Drawing/PreviewManager.cs:209)
UnityEditor.ShaderGraph.Drawing.GraphEditorView.HandleGraphChanges () (at C:/Users/jackpr/AppData/Local/Unity/cache/packages/packages.unity.com/com.unity.shadergraph@1.1.9-preview/Editor/Drawing/Views/GraphEditorView.cs:268)
UnityEditor.ShaderGraph.Drawing.MaterialGraphEditWindow.Update () (at C:/Users/jackpr/AppData/Local/Unity/cache/packages/packages.unity.com/com.unity.shadergraph@1.1.9-preview/Editor/Drawing/MaterialGraphEditWindow.cs:121)
UnityEditor.EditorApplication:Internal_CallUpdateFunctions()