An object reference does not point to an instance of an object. How to fix?

Question

Windows Forms

class ReportClient
{
public SqlConnection connection = new SqlConnection (ConfigurationManager.ConnectionStrings["DefaultConnection"].ConnectionString);
public SqlDataAdapter adapter;

public ReportClient()
{
}

public void DisplayClientData(DataGridView dataGridView1)
{
connection.Open();
DataTable dt = new DataTable();
adapter = new SqlDataAdapter("select * from Client", connection);
adapter.Fill(dt);
dataGridView1.DataSource = dt;
connection.Close();
}

}

Writes that dataGridView passes the value null, but there is data in the table, or where I missed something, before that I used the same piece of code in another program, everything worked without failures