NullReferenceException was unhandled by usercode,
Objectreference not set to an instance of an object
The error message was shown in the following code
Public Sub Proc_CheckedChanged(ByVal sender As System.Object, ByVal e As System.EventArgs)
        Dim CheckBx As  new CheckBox
        Dim checkBx1 as new CheckBox
        Dim dgItem As DataGridItem
        For Each dgItem In datagrid1.Items
            CheckBx = dgItem.FindControl("Checkbox2")
             CheckBx1 = dgItem.FindControl("Checkbox1")
            If CheckBx.Checked Then
               
                CheckBx1.Checked = True
            Else
                 CheckBx1.Checked = False
            End If
        Next
    End Sub
The error was occured in the if condition.

This is a common error which occurs when there is no values are there with the
associated variables/strings/other items which we are using in our code,and 
checking them for the values/assigning those null values to another ones.
In the above code
This error occurs when there is no values checked or the check box checked 
value is "Nothing".