Putting a null value in TempData results in ArgumentNullException on subsequent requests

[DamianEdwards]

Repro steps

1. Create a new MVC application

2. Configure it to use session so that TempData works

   1. Add a reference to Microsoft.AspNetCore.Session
   2. Modify Startup.ConfigureServices to add the session services services.AddSession()
   3. Modify Startup.Configure to add the session middleware app.UseSession()

3. Modify HomeController.Index to insert a null value into TempData:

   public IActionResult Index()
   {
      TempData["WhatWillThisDo"] = null;
      return View();
   }

4. Run the app

Expected

The value at the specified key is cleared.

Actual

The first request succeeds. Subsequent requests fail with the following exception:

System.ArgumentNullException was unhandled by user code
  HResult=-2147467261
  Message=Value cannot be null.
Parameter name: dictionary
  ParamName=dictionary
  Source=System.Private.CoreLib
  StackTrace:
       at System.Collections.Generic.Dictionary`2..ctor(IDictionary`2 dictionary, IEqualityComparer`1 comparer)
       at Microsoft.AspNetCore.Mvc.ViewFeatures.SessionStateTempDataProvider.LoadTempData(HttpContext context)
       at Microsoft.AspNetCore.Mvc.ViewFeatures.TempDataDictionary.Load()
       at Microsoft.AspNetCore.Mvc.ViewFeatures.TempDataDictionary.set_Item(String key, Object value)
       at WebApplication3.Controllers.HomeController.Index() in c:\users\dedward\documents\visual studio 2015\Projects\WebApplication3\src\WebApplication3\Controllers\HomeController.cs:line 13
       at lambda_method(Closure , Object , Object[] )
       at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.<InvokeActionFilterAsync>d__28.MoveNext()
  InnerException: 

[DamianEdwards]

OK this is even worse. It seems to happen if you set it to anything other than a value type, e.g. the same thing happens with the following: TempData["WhatWillThisDo"] = new object();

I'm guessing we don't want that to work, but I'm also guessing we don't want it to fail this way.

[rynowak]

/cc @ajaybhargavb

[danroth27]

We should understand why this is blowing up. If this is something that shouldn't be supported then we should throw a better exception. If bug then fix.