Null reference error when calling a method - asp.net

Question

I have a method called NewBatch_GetAndSetBatchID.
When I call this methode in the Page_Load methode it works perfectly.
But when I call this methode from a button (on the same page), I get:
Null reference exeption Object reference not set to an instance of an object.
It throws an error at:
BatchNoTextBox.Text = (batchID += 1).ToString();
When I debug, I can see that my batchID is filled with a value.
This is my code:
public void NewBatch_GetAndSetBatchID()
{
FormView1.ChangeMode(FormViewMode.Insert);
string connectionString = ConfigurationManager.ConnectionStrings["DBConnection"].ConnectionString;
try
{
using (SqlConnection conn = new SqlConnection(connectionString))
{
SqlCommand sqlCommando = new SqlCommand("SELECT MAX(BatchNo) FROM BM_BrewBatchHeader", conn);
try
{
conn.Open();
batchID = Convert.ToInt32(sqlCommando.ExecuteScalar());
}
catch (Exception)
{
}
finally
{
conn.Close();
}
}
}
catch (SqlException error)
{
}
try
{
TextBox BatchNoTextBox = (TextBox)FormView1.FindControl("BatchNoTextBox");
BatchNoTextBox.Text = (batchID += 1).ToString();
}
catch (Exception)
{
throw;
}
}
What can be the problem here?
I found the solution on another forum:
I have to use the PreRender trigger from the FormView. With the code below it works fine. When I now set the Formview to insert or edit mode the code executes perfectly.
protected void FormView1_PreRender(object sender, EventArgs e)
{
// if the mode is Edit or Insert
if (this.FormView1.CurrentMode == FormViewMode.Edit || this.FormView1.CurrentMode == FormViewMode.Insert)
{
TextBox BatchNoTextBox = (TextBox)FormView1.FindControl("BatchNoTextBox");
BatchNoTextBox.Text = (batchID += 1).ToString();
}
}