Fix Cpu Usage Async Watcher

Author: devizer

Universe.CpuUsage/CpuUsageAsyncWatcher.cs

@@ -43,7 +43,7 @@ public void Stop()
             IsRunning = false;
 #if !NET20 && !NET30 && !NET35 && !NET40 && !NET45
             ContextItem.Value = null;
-            _ContextSwitchListener.Value = null;
+            if (_ContextSwitchListener != null) _ContextSwitchListener.Value = null;
             _ContextSwitchListener = null;
 #endif
             // _Log = null; it is the results