How to solve " null reference exception was unhandled " while deleting gridview row..

Dim vacid As Integer = Convert.ToInt32(DirectCast(GridView1.Rows(e.RowIndex).FindControl("txtVacid"), TextBox).Text)
      Dim myCommand As New SqlCommand
      Dim myConnection = New SqlConnection(ConfigurationSettings.AppSettings("connectionString"))
      Dim update As String
      Dim strsql As String
      myCommand.Connection = myConnection
      'vacid = CInt(GridView1.Rows(GridView1.SelectedRow.RowIndex).Cells(0).Text)
      update = "Delete  from  Pat_Vaccine_Details   where vaccine_id='" & vacid & "'"
      myCommand = New SqlCommand(update, myConnection)
      myConnection.Open()
      myCommand.ExecuteNonQuery()
      myCommand.Connection.Close()
      BindGrid()
  End Sub