skip adding fluent actions menu if it already exists

[xini]

Description

This fixes #888, where in certain situations (e.g. ModelAdmin) the fluent actions menu is added twice, resulting in an error when the form is submitted.

Manual testing steps

see #888

Issues

• Double calls of updateFluentActions lead to RuntimeException #888

Pull request checklist

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[xini]

@tractorcow thanks for the merge. Can this be released as a patch? Cheers