Merge pull request #1236 from peppy/ci-refixes

Fix newly occurring CI issues

osu.Game.Rulesets.Osu/OsuDifficulty/Preprocessing/OsuDifficultyBeatmap.cs

@@ -51,6 +51,7 @@ namespace osu.Game.Rulesets.Osu.OsuDifficulty.Preprocessing
 
                     foreach (OsuDifficultyHitObject h in onScreen)
                     {
+                        // ReSharper disable once PossibleNullReferenceException (resharper not smart enough to understand IEnumerator.MoveNext())
                         h.TimeUntilHit -= latest.DeltaTime;
                         // Calculate reading strain here
                     }

osu.Game/Screens/Play/KeyCounterCollection.cs

@@ -28,6 +28,8 @@ namespace osu.Game.Screens.Play
 
         public override void Add(KeyCounter key)
         {
+            if (key == null) throw new ArgumentNullException(nameof(key));
+
             base.Add(key);
             key.IsCounting = IsCounting;
             key.FadeTime = FadeTime;