How to define Node is null

GoJS
1

I defined the null Node by this statement (context: Angular):
let selectedNode: go.Node = null;
The message I see by the VS Code: Type ‘null’ is not assignable to type ‘Node’.

What is the right way to define the null Node

2

Your TypeScript compiler settings must be including --strictNullChecks.
Please read https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-0.html.