Found markup element with unexpected name 'Cascading AuthenticationState'

I am getting this error on my App.razor:

Found markup element with unexpected name 'CascadingAuthenticationState'. If this is intended to be a component, add a @using directive for it namespace

This is the code I am using

<CascadingAuthenticationState>
    <Router AppAssembly="@typeof(Program).Assembly">
        <Found Context="routeData">
            <RouteView RouteData="@routeData" DefaultLayout="@typeof(MainLayout)" />
        </Found>
        <NotFound>
            <LayoutView Layout="@typeof(MainLayout)">
                <p>Sorry, there's nothing at this address.</p>
            </LayoutView>
        </NotFound>
    </Router>
</CascadingAuthenticationState>

I am using Visual Studio 2019 preview and I can run the application but, why do I have the red line in the Cascading....?

Warning! The .vs folder contains all the data that VS gathered about the projects in your solution, the opened files, debug. You will lose all that!

Close Visual Studios and delete the .vs hidden folder in the root of your solution.

Start up your project and the error's will be gone.