commit /rollback problem

the exeception as followAn unhandled exception of type 'System.NullReferenceException' occurred in system.windows.forms.dll

Additional information: Object reference not set to an instance of an object.

my code as followDim meetError As Boolean = False
Dim conn As Odbc.OdbcConnection
Dim trans As Odbc.OdbcTransaction
Dim cmd As Odbc.OdbcCommand
Dim da As System.Data.ODBC.OdbcDataAdapter
Dim ds As DataSet = New DataSet("dsTable")
Dim dt As DataTable
Dim i As Integer
'
Try
conn = New Odbc.OdbcConnection(connStr)
conn.Open()
cmd = conn.CreateCommand
cmd.Transaction = trans
'
Catch e As Odbc.OdbcException
MsgBox("open error " & e.Message)
meetError = True
Return False
End Try
'
'exec sql
Try
For i = 0 To in_stmt.GetUpperBound(0)
cmd.CommandText = in_stmt(i).value
da = New Odbc.OdbcDataAdapter(cmd.CommandText, conn)
cmd.ExecuteNonQuery()
'
Try
dt = New DataTable(i.ToString)
da.Fill(dt)
If dt.Columns.Count > 0 Then
ds.Merge(dt)
End If
Catch ex As Exception
MsgBox("merge error " & ex.Message)
End Try
Next
Catch ex As Odbc.OdbcException
MsgBox("execute error " & ex.Message)
meetError = True
End Try
'
If meetError Then
trans.Rollback()
Else
trans.Commit()
End If
'
'close connectoin
'
grd.DataSource = ds
Try
conn.Close()
conn = Nothing
Catch e As Odbc.OdbcException
MsgBox("close error " & e.Message)
End Try
Return True
thanks for help.I think the problem is
can da.fill(ds) after trans.begin but before trans.commit.
thanks