title: await with httpClient.GetAsync(url) and getting error System.NullReferenceException was unhandled
body: <p>I am using <code>VS 2012</code> and set the target <code>framework 4.0.</code></p>
  
  <pre><code>public async Task&lt;HttpResponseMessage&gt; GetResponse(Uri url)             
  {                 
      this.httpClient.DefaultRequestHeaders.Accept.Clear();                 
      HttpResponseMessage response = await this.httpClient.GetAsync(url);                 
      response.EnsureSuccessStatusCode();             
  }
  </code></pre>
  
  <p>Error:</p>
  
  <pre><code>System.NullReferenceException was unhandled
  HResult=-2147467261
  Message=Object reference not set to an instance of an object.
  Source=Microsoft.CompilerServices.AsyncTargetingPack.Net4
  StackTrace:
  Server stack trace: at System.Web.ThreadContext.AssociateWithCurrentThread(Boolean  setImpersonationContext) at System.Web.HttpApplication.OnThreadEnterPrivate(Boolean setImpersonationContext) at System.Web.LegacyAspNetSynchronizationContext.CallCallbackPossiblyUnderLock(SendOrPostCallback callback, Object state) at System.Web.LegacyAspNetSynchronizationContext.CallCallback(SendOrPostCallback callback, Object state) at System.Web.LegacyAspNetSynchronizationContext.Post(SendOrPostCallback callback, Object state) at System.Runtime.CompilerServices.TaskAwaiter.&lt;&gt;c__DisplayClassa.&lt;OnCompletedInternal&gt;b__0(Task param0)
          Exception rethrown at [0]: at System.Runtime.CompilerServices.AsyncMethodBuilderCore.&lt;ThrowAsync&gt;b__1(Object state)
             at System.Threading.QueueUserWorkItemCallback.WaitCallback_Context(Object state)
             at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
             at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
             at System.Threading.QueueUserWorkItemCallback.System.Threading.IThreadPoolWorkItem.ExecuteWorkItem()
             at System.Threading.ThreadPoolWorkQueue.Dispatch()
             at System.Threading._ThreadPoolWaitCallback.PerformWaitCallback()
        InnerException: 
      -------------------------------------------------
  </code></pre>
  
  <p>I have installed <code>"Microsoft.CompilerServices.AsyncTargetingPack</code>" from <code>NuGet</code>, but still getting this error. Any suggestion.</p>