BREAKING: Replace SetOnce Get/Set with Value, #1069

[paulirwin]

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Replaces Get and Set on the SetOnce<T> type with a new Value property, marks old methods as deprecated for removal.

Fixes #1069

Description

In addition to the above, this also enables nullable in the file, and adds more meaningful exceptions in cases where otherwise it could possibly throw a NRE.

[NightOwl888]

Thanks for the PR. Looks good to merge.