"The ConnectionString property has not been initialized." with json file

I'm trying to get the connectionstring from the json-file to the DAL-class library with:

private static IConfigurationRoot config;
private static string connectionString;

public UserDAL()
{
    config = new ConfigurationBuilder().AddJsonFile("appsettings.json").Build();
    connectionString = config.GetConnectionString("DefaultString");
}

In the json-file:

{
  "ConnectionStrings": {
    "DefaultString": "theconnectionstring"
  },
  "Logging": {
    "LogLevel": {
      "Default": "Information",
      "Microsoft": "Warning",
      "Microsoft.Hosting.Lifetime": "Information"
    }
  },
  "AllowedHosts": "*"
}

The error happens when I try to open the connection, so con.Open();