.Net can't connect to mariadb database in docker container

I’m trying to connect to my Mariadb database that’s inside a Docker container. I’ve looked at Microsoft’s documentation and searched on Google to find out why I’m getting an error message saying that there’s nothing to retrieve from the database. It’s strange because when I try to debug my code, it doesn’t execute the lines that check the database. The error happens at the line "conn.Open();" and then it just stops. I can also see in Docker that it’s getting a connection, so the connection string should be correct. Can anyone explain what I’m doing wrong?

This is the error message:

System.InvalidCastException: ‘Object cannot be cast from DBNull to
other types.’

Here is my code:

using MySql.Data.MySqlClient;

namespace DockerBase
{
    internal class UserDB
    {
        public bool ValidateUser(string username, string password)
        {
            try
            {
                MySqlConnectionStringBuilder builder = new MySqlConnectionStringBuilder();
                builder.Server = "localhost";
                builder.Port = 3306;
                builder.UserID = "root";
                builder.Password = "rootpassword";
                builder.Database = "userDB";

                using (MySqlConnection conn = new MySqlConnection(builder.ConnectionString))
                {
                    conn.Open();
                    string sql = "SELECT COUNT(*) FROM users WHERE username = @username AND password = @password;";
                    int result = 0;

                    using (MySqlCommand cmd = new MySqlCommand(sql, conn))
                    {
                        cmd.Parameters.AddWithValue("@username", username);
                        cmd.Parameters.AddWithValue("@password", password);
                        result = Convert.ToInt32(cmd.ExecuteScalar());
                    }

                    bool isValid = (result > 0);
                    return isValid;
                }
            }
            catch (MySqlException e)
            {
                Console.WriteLine("Error connecting to MySQL database: " + e.Message);
                return false;
            }
        }
    }
}

And here is my dockerfile also

FROM mariadb:latest

ENV MYSQL_DATABASE=userDB
ENV MYSQL_USER=myuser
ENV MYSQL_PASSWORD=mypassword
ENV MYSQL_ROOT_PASSWORD=rootpassword

# Copy SQL scripts
COPY createUser.sql /docker-entrypoint-initdb.d/

EXPOSE 3306

And also my sql code:

USE userDB;

DROP TABLE IF EXISTS users;

CREATE TABLE users (
    username VARCHAR(50) NOT NULL,
    password VARCHAR(50) NOT NULL,
);

INSERT INTO users (username, password) VALUES ('admin', 'admin');

In the terminal in Dockerhub I can see that a connection is being made:

Aborted connection 3 to db: ‘userDB’ user: ‘root’ host: ‘REMOVED’ (Got an error reading communication packets)