Null Reference Exception Thrown while using connectionstring inside static classes

Question

In my projects, I used connection string inside non-static classes, and It works fine. But now I'm developing a project that uses connection string inside static classes. In the code below the Catch block will throw an error as shown below:

NullReferenceException: Object reference not set to an instance of an object.

If I comment out this line "Throw" it works fine, but I want to understand what went wrong in my code.

DataAccess class:

public class DataAccess
{
    static string connstr =  ConfigurationManager.ConnectionStrings["DbSystem2018"].ConnectionString;

    public static object GetSingleAnswer(string sql, List<SqlParameter> PList)
    {
        object obj = null;

        SqlConnection conn = new SqlConnection(connstr);

        try
        {
            conn.Open();
            SqlCommand cmd = new SqlCommand(sql, conn);

            if (PList != null)
            {
                foreach (SqlParameter p in PList)
                    cmd.Parameters.Add(p);
            }

            obj = cmd.ExecuteScalar();
        }
        catch (Exception ex)
        {
            throw; // THIS LINE THROW THE NullReferenceException Error
        }
        finally
        {
            conn.Close();
        }

        return obj;
    }
}

The problem is with using static connection string field inside static classes. So, is there any way to solve this issue without using any non static public classes?

Updated:

Please notice the following:

1. I don't want to use non-static methods, my question is how we can fix this using static methods.

2. When we use static field, we initialize it right away:

   static string v1 = "hello";
   

   but why the initialization for the following code throw null before returning the actual connection string?

   static string connstr = ConfigurationManager.ConnectionStrings["DbSystem2018"].ConnectionString;
   

By the way, the connection string inside the web.config is fine.

Answer 1

This:

System.Configuration.Configuration config = System.Web.Configuration.WebConfigurationManager.OpenWebConfiguration(null);
if (config.ConnectionStrings.ConnectionStrings.Count > 0)
{
    var connString = config.ConnectionStrings.ConnectionStrings[nameOfConnectionString].ToString();
    return connString;
}

Is copy paste from: https://www.codeproject.com/Articles/1012286/Read-web-config-Settings

The problem is that you're accessing a config file. Those have to be converted to an object before they are accesible. This is what the code above does.