gridview selected row value to textbox on another form

I'm not sure why there's an error here unless there are no selected rows or the value of the property is null when the code runs.

However, if you want to track the selected rows dynamically, then you'll need to use an event. Something like this (off top of my head so may be errors):

// Form2

private void add_item_Load(object sender, EventArgs e) 
{ 
   Item_setup f1 = (Item_setup)Application.OpenForms["Item_setup"];    
   f1.dataGridView1.SelectionChanged += Form1_DGV_SelectionChanged;           
   // call handler manually to get initial value
   Form1_DGV_SelectionChanged(f1.dataGridView1, EventArgs.Empty); 
}

// add this handler manually to Form2, not through VS designer
private void Form1_DGV_SelectionChanged(object sender, EventArgs e)
{
   DataGridView dgv = (DataGridView)sender;
   DataGridViewSelectedRowCollection selRows = dgv.SelectedRows; 
   if (selRows != null && selRows.Count > 0 && selRows[0].Cells[0].Value != null)
   {
     textBox1.Text = selRows[0].Cells[0].Value.ToString(); 
   }
   else
   {
     textBox1.Text = "";
   }    
}

First change the Modifiers property of the DGV (say dataGridView1) on Form1 from private to internal.

Then on Form2 use this code to get the SelectedRows collection:

Form1 f1 = (Form1)Application.OpenForms["Form1"];
DataGridViewSelectedRowCollection selRows = f1.dataGridView1.SelectedRows;

If the value you want is in the first column of the first (or only) selected row, you can then do:

textBox1.Text = selRows[0].Cells[0].Value.ToString();