check multiple textboxes is emty oe not at a time

Question

My textBoxes Is contained on panel3 and panel3  is above  on panel2

I want validate  textboxes my code is

 

public void ClearTextBoxes(Control.ControlCollection cc)

{

foreach (Control ctrl in cc)

{

TextBox tb = ctrl as TextBox;

if (tb == null)

messagebox.show("Empty Textbox Found");

}

}

 but It Throws an error -"Object refference not set an instance of an object"