title: Could not load file or assembly netstandard when I add migration
body: <p>I have an ASP.Net Core 3.0 solution with 2 projects: Web (start project) and Core (class library). I try to create a migration using Package Manager Console:</p>
  
  <blockquote>
    <p>add-migration Initial -Project Core -StartupProject Web</p>
  </blockquote>
  
  <p>and get the following error:</p>
  
  <blockquote>
    <p>Could not load file or assembly 'netstandard, Version=2.1.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51'. The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)</p>
  </blockquote>
  
  <p>Both projects have Microsoft.EntityFrameworkCore.SqlServer and Microsoft.EntityFrameworkCore.Tools in references. That's how I create a connection in Startup.cs:</p>
  
  <pre><code>public void ConfigureServices(IServiceCollection services)
  {
      var connection = Configuration.GetConnectionString("DefaultConnection");
      services.AddDbContext&lt;ApplicationContext&gt;(options =&gt;
      options.UseSqlServer(connection));
      services.AddControllersWithViews();
  }
  </code></pre>
  
  <p>and my connectionstring in appsettings.json:</p>
  
  <pre><code>"ConnectionStrings": {
          "DefaultConnection": "Server=(localdb)\\mssqllocaldb;Database=FinApp;Trusted_Connection=True;"
      }
  </code></pre>
  
  <p>Here's my ApplicationContext.cs:</p>
  
  <pre><code>public class ApplicationContext : DbContext
  {
      public DbSet&lt;Account&gt; Accounts { get; set; }
      public DbSet&lt;Category&gt; Categories { get; set; }
      public DbSet&lt;Currency&gt; Currencies { get; set; }
      public DbSet&lt;Operation&gt; Operations { get; set; }
      public DbSet&lt;User&gt; Users { get; set; }
  
      public ApplicationContext()
      {
      }
  }
  </code></pre>
  
  <p>Does anyone have the same issue?</p>