Fix possible idleTracker nullref

osu.Game/Input/IdleTracker.cs

@@ -0,0 +1,41 @@
+// Copyright (c) 2007-2018 ppy Pty Ltd <contact@ppy.sh>.
+// Licensed under the MIT Licence - https://raw.githubusercontent.com/ppy/osu/master/LICENCE
+
+using osu.Framework.Graphics;
+using osu.Framework.Input;
+using osu.Framework.Input.Bindings;
+using osu.Framework.Input.Events;
+
+namespace osu.Game.Input
+{
+    public class IdleTracker : Component, IKeyBindingHandler<PlatformAction>
+    {
+        private double lastInteractionTime;
+        public double IdleTime => Clock.CurrentTime - lastInteractionTime;
+
+        private bool updateLastInteractionTime()
+        {
+            lastInteractionTime = Clock.CurrentTime;
+            return false;
+        }
+
+        public bool OnPressed(PlatformAction action) => updateLastInteractionTime();
+
+        public bool OnReleased(PlatformAction action) => updateLastInteractionTime();
+
+        protected override bool Handle(UIEvent e)
+        {
+            switch (e)
+            {
+                case KeyDownEvent _:
+                case KeyUpEvent _:
+                case MouseDownEvent _:
+                case MouseUpEvent _:
+                case MouseMoveEvent _:
+                    return updateLastInteractionTime();
+                default:
+                    return base.Handle(e);
+            }
+        }
+    }
+}