A TabItem can be either of type Tab or TabSeparator. When working with TabItem objects you should make sure you have the right type.
Infragistics.WebUI.WebSchedule.UltraWebTab.Tab t;
Infragistics.WebUI.WebSchedule.UltraWebTab.TabSeparator ts;
if (UltraWebTab1.Tabs[0].GetType().Equals(typeof(Infragistics.WebUI.WebSchedule.UltraWebTab.Tab)))
Target Platforms: Windows 10, Windows 8.1, Windows 8, Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2