Account for sudden map deletion

[Emik03]

Current stable build can cause ArgumentOutOfRangeException if user deletes a chart, sometimes.

[1:55:21] - RUNTIME - DEBUG: Deleted map: 4f633542553b5667186a8cafad30ab2a (#5023) in the cache
[1:55:21] - RUNTIME - ERROR: System.ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection. (Parameter 'index')
   at System.Collections.Generic.List`1.get_Item(Int32 index)
   at Quaver.Shared.Screens.Selection.SelectionScreen.<>c__DisplayClass77_0.<OnMapsetDeleted>b__0() in D:\QuaverDeploy\Quaver\Quaver.Shared\Screens\Selection\SelectionScreen.cs:line 914
   at Quaver.Shared.Scheduling.ThreadScheduler.<>c__DisplayClass1_0.<Run>b__0() in D:\QuaverDeploy\Quaver\Quaver.Shared\Scheduling\ThreadScheduler.cs:line 38