title: Unable to find package Microsoft.NETCore.App with version(>=3.0.0)
body: <p>I am trying to migrate my WPF(.net framework) project to WPF(.net core 3).
  So i have installed <a href="https://marketplace.visualstudio.com/items?itemName=BrianLagunas.ConvertProjectToNETCore3" rel="noreferrer">this Visual Studio Extension</a> and i m now able to create a new Wpf(.net core) project , but the problem starts when i add a nuget package ! , VS throws me this error :</p>
  
  <pre><code>Unable to find package Microsoft.NETCore.App with version (&gt;= 3.0.0-preview6-27730-01)
  - Found 69 version(s) in nuget.org [ Nearest version: 3.0.0-preview5-27626-15 ]
  - Found 0 version(s) in Microsoft Visual Studio Offline Packages    TestwpfCore C:\Users\sintware\source\repos\TestwpfCore\TestwpfCore\TestwpfCore.csproj   1   
  </code></pre>