How to reproduce:
1. Open the attached project named "Bug Repro Project.zip"
2. Open the "Can't Add Keyword" shaderGraph file. Located in - Assets/Shaders
3. In the Graph's Blackboard Add a new Enum Keyword

Expected result: Enum Keyword is added or an appropriate error message is thrown informing why the Keyword could not be added
Actual result: NullReferenceException is thrown after the Keyword is created. ShaderGraph becomes unusable after this point, throwing ArgumentNullException while trying to interact with the graph

Reproducible with: 10.2.2(2020.2.3f1), 11.0.0(2021.1.0b3), 12.0.0(2021.2.0a2)
Could not test with: 2018.4(ShaderGraph was in early preview), 7.5.2(Graphs required for the repro break on downgrade)

Alternative repro steps:
1. Open the "Can't Add Sub Graph" shaderGraph file. Located in - Assets/Shaders
2. Try to add the Sub Graph named "Toon Shading Modular"

Notes:
1. Seems to occur with complex nested graphs, when there is a high number of keywords and properties used