[Ruby][SG] Fix exception being thrown on undo-ing action of "Add Input to Blackboard Category"

[Nightmask3]

---

Purpose of this PR

Fixes bug: https://fogbugz.unity3d.com/f/cases/1348910/

Broken behavior:

WHZjAVBQaw.mp4

Fixed behavior:

rW6l5QO71q.mp4

---

Testing status

• Verified bug behavior no longer repros
• Tested selection and undo/redo interaction as those behaviors were entangled with this code

---

Notes to reviewers

• Please do make sure to validate the selection stack and undo/redo behavior as there is some halo effect with these changes to that
  There is one known issue where trying to undo selection to an empty selection stack causes it to skip the empty selection step and go one selection step previous to that, but it is not consistent, and has been deemed okay to ship with, will be investigated as a separate bug.

[github-actions]

Hi! This comment will help you figure out which jobs to run before merging your PR. The suggestions are dynamic based on what files you have changed.
Link to Yamato: https://yamato.cds.internal.unity3d.com/jobs/902-Graphics
Search for your PR branch using the sidebar on the left, then add the following segment(s) to the end of the URL (you may need multiple tabs depending on how many packages you change)

Shader Graph
/.yamato%252Fall-shadergraph.yml%2523PR_ShaderGraph_trunk
Depending on your PR, you may also want
/.yamato%252Fall-shadergraph_builtin_foundation.yml%2523PR_ShaderGraph_BuiltIn_Foundation_trunk
/.yamato%252Fall-shadergraph_builtin_lighting.yml%2523PR_ShaderGraph_BuiltIn_Lighting_trunk

Depending on the scope of your PR, you may need to run more jobs than what has been suggested. Please speak to your lead or a Graphics SDET (#devs-graphics-automation) if you are unsure.

[jessebarker]

LGTM

[bencloward]

I tested the repro steps locally and was able to repro the error. Then I switched to this branch and followed the steps again, but this time the error did not occur. Nice work!