Reproduction steps:
1. Import the provided custom package "New.unitypackage"
2. In the Project window under "Shaders/Breathing" open "breathingGuide01" in Shader Graph
3. Change Vector 1 value to 0.5 and undo the change with CTRL + Z

Expected result: Main Preview window does not freeze when undoing changes
Actual result: Main Preview window freezes when undoing changes

Reproduces on: Shader Graph 7.1.1 (2019.3.0b5), Shader Graph 7.3.1 (2019.3.9f1), Shader Graph 8.0.1 (2020.1.0b5, 2020.2.0a7)
Could not test on 2018.4.20f1 and 2019.3.0b4 (Shader Graph 7.0.1) due to imported Shader Graph console errors
Could not test on 2017.4.39f1 (No package)

Notes:
-Reopening the Shader Graph solves the issue