For me it was that the connectionString in Startup.cs was null in:
services.AddDbContext<ApplicationDbContext>(options => options.UseSqlServer(Configuration.GetConnectionString("DefaultConnection")));
and it was null because the application was not looking into appsettings.json for the connection string.
Had to change Program.cs to:
public static void Main(string[] args)
{
BuildWebHost(args).Run();
}
public static IWebHost BuildWebHost(string[] args) =>
WebHost.CreateDefaultBuilder(args)
.ConfigureAppConfiguration((context, builder) => builder.SetBasePath(context.HostingEnvironment.ContentRootPath)
.AddJsonFile("appsettings.json").Build())
.UseStartup<Startup>().Build();