vb.net将DataGridView赋值给二维数组出错,该怎么解决

vb.net将DataGridView赋值给二维数组出错
运行时报错：未将对象引用设置到对象的实例。什么问题？
[code=VB.NET][/code]
 Dim dwm_row, dwm_cloumn As Integer
  dwm_row = bom_mx_dgv.Rows.GetRowCount(DataGridViewElementStates.Displayed) - 1
  dwm_cloumn = bom_mx_dgv.Columns.GetColumnCount(DataGridViewElementStates.Displayed) - 1

  Dim mx_dt(,)
  For i = 0 To dwm_row
  For j = 0 To dwm_cloumn
  mx_dt(i, j) = bom_mx_dgv.Rows(i).Cells(j).Value
  Next
  Next


[解决办法]
1.有没有隐藏列
2.有没有新增行
如里有你上面的就会错