`GameplayBeatmap` has to be used instead of the normal bindable
`Beatmap`, beecause the former uses osu!-specific hitobjects, while
the latter returns convert objects (i.e. `ConvertSlider`s).
Similarly, the mod has to be fetched from the player instead of the
global bindable, as `Player` has its own cloned instance of the mod, to
which the beatmap is applied. The global bindable instance does not have
`FirstObject` set.
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