Object reference not set to an instance of an object.

Where is the code in your opening post running from?

If it's running from the form which launched the child form containing the DGV, then you need to store a reference to the child form in a field of the parent form so that you can access it later:-

private calculPolita calculCasco;

// in button click handler

calculCasco = new calculPolita();
calculCasco.MdiParent  = this; // or whatever you're doing here
calculCasco.Show();

// in some other method of parent form

DataGridView dgv = calculCasco.calculGridView;

If it's running from some other form, then probably the easiest thing to do is to introduce an internal static field into the child form class which stores a reference to it:

class calculPolita : Form
{
    internal static calculPolita Me;
    
    public calculPolita() // constructor
    {  
        InitializeComponent();
        Me = this;
    }

    // rest of code
}

Another form can then access the DGV as follows:

DataGridView dgv = calculPolita.Me.calculGridView; 

Hi...

check the condition -

if(calculPolita == null)
{
  calculPolita = new CalculPolita(); // classname
}
else
{
}
--------
Hope this helps