The second call to WithViewData throws NullReferenceException

This code:
```csharp
const string pageName = "pageName";

_sut.GetByPageName(pageName)
.Should()
.BeViewResult()
.WithViewData("PageName", pageName)
.WithViewData("Roles", _roles) // this call fails!
.WithViewData("FieldName", null);
```

causes:
```
System.NullReferenceException
HResult=0x80004003
Message=Object reference not set to an instance of an object.
Source=FluentAssertions.Mvc5
StackTrace:
at FluentAssertions.Mvc.ViewResultBaseAssertions`1.WithViewData(String key, Object expectedValue, String reason, Object[] reasonArgs)
at Tests.Controllers.HelpEditor.HelpEditorControllerTest.GetByPageName_Should_return_view_for_specified_pageName() in ...\....Tests\Controllers\HelpEditor\HelpEditorControllerTest.cs:line 48

```

Start with ViewResultBaseAssertions.WithViewData, using the stack trace and the HelpEditorControllerTest.GetByPageName_Should_return_view_for_specified_pageName test as entry points. Reproduce the chained assertions and verify that successive WithViewData calls, including the final null value, complete without a NullReferenceException.

Written by the indexing model from the issue text.