[ShaderGraph] [2022.1] Fix for exception thrown when changing packages with embedded subgraphs

[cdxntchou]

Purpose of this PR

Fixing an issue I ran into, where sometimes the subgraph asset is destroyed, and we don't remove it from the cached list before attempting to access it. This PR just adds a check to ignore the cache entry if it has been destroyed.

As far as I can tell, this is triggered when I have a package mapped to a repo, that contains a subgraph, and I sync the repo (with a subgraph asset deletion) while Unity is running. ShaderGraph seems to run this code before the removing the deleted asset from the list, resulting in an exception being thrown when the asset is accessed -- the exception borks ShaderGraph, making it unoperational.

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Testing status

Tested against the repro steps above (changing package version). The PR fixes the issue.

Yamato:
ShaderGraph PR Job: https://yamato.cds.internal.unity3d.com/jobs/902-Graphics/tree/sg%252Ffix%252Fsearch-window-exception/.yamato%252Fall-shadergraph.yml%2523PR_ShaderGraph_2021.2/8407608/job

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Comments to reviewers

Notes for the reviewers you have assigned.

[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_2021.2
Depending on your PR, you may also want
/.yamato%252Fall-shadergraph_builtin_foundation.yml%2523PR_ShaderGraph_BuiltIn_Foundation_2021.2
/.yamato%252Fall-shadergraph_builtin_lighting.yml%2523PR_ShaderGraph_BuiltIn_Lighting_2021.2

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