get all row with checkboxcolumn checked

How I can loop through all row of my grid and detect only row with checkbox column checked ?

i use this solution and got error :  

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Error:

"System.NullReferenceException: 'Object reference not set to an instance of an object.'Telerik.WinControls.UI.GridViewCellInfo.Value.get returned null."
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Code:

IList<GridViewRowInfo> gridRows = new List<GridViewRowInfo>();
foreach (GridViewRowInfo rowInfo in radGridView.ChildRows)
{
    bool isChecked = (bool)rowInfo.Cells["CheckboxColumn"].Value;
    if (isChecked == true)
    {
        gridRows.Add(rowInfo);
    }
}