NullReferenceExceptions thrown on removing connected slot to the other node in SubGraphoutputs window. Unable to create any action like create a new node or delete the existing node.

Steps to repro:
1. Create a new project.
2. Windows > Package Manager > Shader Graph > Install.
3. Right click on Project Window > Create > Shader.
4. Select Sub Graph > Double click on the newly created Sub Graph.
5. Add slots in SubGraphOutputs window.
6. Create Color node and connect to the Output 1 of SubGraphOutputs window.
7. Remove slot from SubGraphOutputs window

Actual Result:
NullReferenceExceptions in the console.

Expected result:
No errors in the console.

Reproducible in:
2019.1.0a13, 2019.1.0a10.

Working Fine in:
2019.1.0a9, 2018.3.0f2(49866f0a5d12)

Environment:
Windows & Mac.