System.nullReferenceException，对象引用未设置为对象的实例

Question

我想匹配一列dataGridView（dataGridViewLAS）和一个列表（TransitTime），但是这种例外情况不允许！

我的代码：

 private void fillDataGridViewLAS_CellContentClick(List<TPorosity> 
lstPorosity)
        {
            if (lstPorosity == null)
                return;
            foreach (TPorosity Porosity in lstPorosity)
            {
                DataGridViewLAS.Rows.Add(TTools.Depth, Porosity.Density, 
Porosity.Neutron, TPorosity.TransitTime);

            }
        }
    private void BtnOK_Click(object sender, EventArgs e)
    {
        for (int i = 0; i < DataGridViewLAS.Rows.Count; i++)
        {
            if (DataGridViewLAS[3, i].Value != null)
            {
                DataGridViewLAS[3, i].Value = TPorosity.TransitTime[i];
            }
        }
    }