ASP.NET Core 2 Error when get connection string

I am using ASP.NET Core 2 project file <Project Sdk=”Microsoft.NET.Sdk.Web”> <PropertyGroup> <TargetFramework>netcoreapp2.1</TargetFramework> </PropertyGroup> <ItemGroup> <PackageReference Include=”Microsoft.AspNetCore.App” /> <PackageReference Include=”DevExtreme.AspNet.Data” Version=”2.4.4″ /> <PackageReference Include=”DevExtreme.AspNet.Core” Version=”19.1.6″ /> <PackageReference Include=”Microsoft.EntityFrameworkCore” Version=”2.1.11″ /> <PackageReference Include=”Microsoft.EntityFrameworkCore.SqlServer” Version=”2.1.11″ /> <PackageReference Include=”Microsoft.EntityFrameworkCore.Tools” Version=”2.1.11″> <PrivateAssets>all</PrivateAssets> <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets> </PackageReference> <PackageReference Include=”Microsoft.VisualStudio.Web.BrowserLink” Version=”2.1.1″ /> <PackageReference Include=”BuildBundlerMinifier” Version=”2.9.406″ /> </ItemGroup> <ItemGroup> <DotNetCliToolReference…