If you’re using the TabContainer, which is part of the Ajax Control Toolkit and have noticed that when you remove a tab you have issues accessing the values from a DropDownList using .SelectedItem.Value or .SelectedValue then below is the solution:
Instead of using the below to remove a tab:
TabContainer1.Tabs.Remove(TabPanel1)
Use this code to remove the tab instead:
TabContainer1.Tabs.Item(0).Visible = False
As you can see, you need to state the index rather than the id, but just remember that indexes start at 0.