[BUG] Playing with vocals doesn't allow getting past difficulty select

[muskit]

To reproduce:

1. Have a microphone player set up
2. Go to Quickplay, choose a song
3. Choose the mode

• Solo/harmony lets us continue to the next step
• Sit Out raises an exception in the same fashion as the next step

4. Choose the difficulty for vocals. You'll be unable to proceed, as an exception is raised:

ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection.
Parameter name: index
System.Collections.Generic.List`1[T].get_Item (System.Int32 index) (at <88e4733ac7bc4ae1b496735e6b83bbd3>:0)
YARG.UI.DifficultySelect.IncreasePlayerIndex () (at Assets/Script/UI/DifficultySelect.cs:189)
YARG.UI.DifficultySelect.Next () (at Assets/Script/UI/DifficultySelect.cs:167)
YARG.UI.DifficultySelect.ClickOption (YARG.UI.GenericOption option) (at Assets/Script/UI/DifficultySelect.cs:140)
YARG.UI.GenericOption.MouseClick () (at Assets/Script/UI/GenericOption.cs:27)
UnityEngine.Events.InvokableCall.Invoke () (at <ba783288ca164d3099898a8819fcec1c>:0)
UnityEngine.Events.UnityEvent`1[T0].Invoke (T0 arg0) (at <ba783288ca164d3099898a8819fcec1c>:0)
UnityEngine.EventSystems.EventTrigger.Execute (UnityEngine.EventSystems.EventTriggerType id, UnityEngine.EventSystems.BaseEventData eventData) (at Library/PackageCache/com.unity.ugui@1.0.0/Runtime/EventSystem/EventTrigger.cs:220)
UnityEngine.EventSystems.EventTrigger.OnPointerClick (UnityEngine.EventSystems.PointerEventData eventData) (at Library/PackageCache/com.unity.ugui@1.0.0/Runtime/EventSystem/EventTrigger.cs:277)
UnityEngine.EventSystems.ExecuteEvents.Execute (UnityEngine.EventSystems.IPointerClickHandler handler, UnityEngine.EventSystems.BaseEventData eventData) (at Library/PackageCache/com.unity.ugui@1.0.0/Runtime/EventSystem/ExecuteEvents.cs:57)
UnityEngine.EventSystems.ExecuteEvents.Execute[T] (UnityEngine.GameObject target, UnityEngine.EventSystems.BaseEventData eventData, UnityEngine.EventSystems.ExecuteEvents+EventFunction`1[T1] functor) (at Library/PackageCache/com.unity.ugui@1.0.0/Runtime/EventSystem/ExecuteEvents.cs:272)
UnityEngine.EventSystems.EventSystem:Update() (at Library/PackageCache/com.unity.ugui@1.0.0/Runtime/EventSystem/EventSystem.cs:514)

Occurs with single and multiple microphones, bots or controlled players. Debugging in Unity Editor, I see that DifficultySelect is stuck at -1, and an exception occurs when trying to access PlayerManager.players[playerIndex].