Prevents potential multiple evaluations of enumerable.
Regressed with PlayfieldAdjustmentContainer changes.
# Conflicts: # osu.Game.Rulesets.Catch/UI/CatchRulesetContainer.cs # osu.Game.Rulesets.Mania/Edit/ManiaEditRulesetContainer.cs # osu.Game.Rulesets.Osu/Edit/OsuEditRulesetContainer.cs # osu.Game/Rulesets/UI/RulesetContainer.cs