Fixes duplicate tree view add button for closed Controlled Vocabularies

[aseyedia]

References

Fixes #3325

Description

The Controlled Vocabulary tree view was displaying two "Add" buttons, one enabled and one disabled, for closed Controlled Vocabularies.

You can read more here.

Instructions for Reviewers

Minor change. I just removed one of the excess buttons and moved the conditional logic to the [disabled] parameter in the button class.

To test it, try opening up a closed Controlled Vocabulary tree view and observing the "Add" button to make sure it's disabled and that there's only one of them.

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