Fix NRE when trying to start the stopwatch (#747)

* Fix NRE when trying to start the stopwatch
* Make Stopwatch property threadstatic

Author: rjmholt

src/PowerShellEditorServices/Utility/ExecutionTimer.cs

@@ -50,7 +50,7 @@ public static ExecutionTimer Start(
             [CallerLineNumber] int callerLineNumber = -1)
         {
             var timer = new ExecutionTimer(logger, message, callerMemberName, callerFilePath, callerLineNumber);
-            t_stopwatch.Start();
+            Stopwatch.Start();
             return timer;
         }
 
@@ -93,6 +93,6 @@ public void Dispose()
                 callerLineNumber: _callerLineNumber);
         }
 
-        private Stopwatch Stopwatch => t_stopwatch ?? (t_stopwatch = new Stopwatch());
+        private static Stopwatch Stopwatch => t_stopwatch ?? (t_stopwatch = new Stopwatch());
     }
 }