title: C# compiler throws Language Version (LangVersion) reference error "Invalid 'nullable' value: 'Enable' for C# 7.3"
body: <p>I have solution with couple .NET Standard projects in all I wanted to enable c# 8 and nullable like below:</p>
  <pre><code>&lt;PropertyGroup&gt;
      &lt;TargetFramework&gt;netstandard2.1&lt;/TargetFramework&gt;
      &lt;LangVersion&gt;8.0&lt;/LangVersion&gt;
      &lt;Nullable&gt;enable&lt;/Nullable&gt;
    &lt;/PropertyGroup&gt;
  </code></pre>
  <p><strong>Note: These settings are found in your .csproj file.</strong></p>
  <p>The problem is that some projects are compiling fine and some have error:</p>
  <blockquote>
  <p>Invalid 'nullable' value: 'Enable' for C# 7.3. Please use language
  version 'preview' or greater</p>
  </blockquote>
  <p>I have Visual Studio 16.2 Preview 2 and .NET Core 3 Preview 6. Is this a bug in preview or I'm doing something wrong?</p>