Fix crash on load for Excel sheets with Datasets

[IsakNaslundBh]

NOTE: Depends on

Issues addressed by this PR

Closes #365

Only call UpdateEnumValues for relevant caller types and add null check as extra protection

Test files

in fresh Excelsheet:
Add dataset (any dataset)
Add some other method
Save and close
Reopen sheet

Changelog

Additional comments

[IsakNaslundBh]

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[FraserGreenroyd]

Code changes are sensible and good protections added.

[FraserGreenroyd]

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