Clear all dynamically generated usercontrol in mainform on buttonclick on form 2

Question

On my main Form I have a panel with dynamically generated UserControl in it. I want to clear all the UserControls on button click on another UserControl.

Here's what I've tried:

private Form1 mainForm = null;
public ProductCategoryUC(Form1 callingForm)
{
    mainForm = callingForm as Form1;
    InitializeComponent();
}

public ProductCategoryUC(string Text, Image imagee)
{
    InitializeComponent();
    categorybtn.LabelText = Text;
    categorybtn.Image = imagee;
}

private void categorybtn_Click(object sender, EventArgs e)
{
    foreach (Control item in mainForm.productsContainer1.Controls.OfType<UserControl>())
    {
        item.Controls.Clear();
    }
}

But getting this error:

System.NullReferenceException'