'The SelectCommand property has not been initialized before calling 'Fill'. ' error

I'm trying to have a DataGridView in my Windows Forms showing data from the database, but it doesn't want to run.

namespace DentalClinicManagementSystem
{
    class MyPatient
    {
        public void AddPatient(string query)
        {
            ConnectionString MyConnection = new ConnectionString();

            SqlConnection Con = MyConnection.GetCon();
            SqlCommand cmd = new SqlCommand();
            cmd.Connection = Con;

            Con.Open();
            cmd.CommandText = query;
            cmd.ExecuteNonQuery();
            Con.Close();
        }

        public DataSet ShowPatient(string query)
        {
            ConnectionString MyConnection = new ConnectionString();

            SqlConnection Con = MyConnection.GetCon();
            SqlCommand cmd = new SqlCommand();
            cmd.Connection = Con;
            cmd.CommandText = query;

            SqlDataAdapter sda = new SqlDataAdapter(cmd);
            DataSet ds = new DataSet();

            sda.Fill(ds);

            return ds;
        }
    }
}