When PanelSet Alerts are enabled (PanelSet.AlertsEnabled = true), this message could appear if
a PanelSet.Splitter cannot locate the Panel.ID referenced by the
Splitter.ResizePanel1ID or ResizePanel2ID properties.
If this is occurring and a syntax error cannot be found, then the most likely reason is that
the ASP.NET Framework has prefixed the ID of the Panel at run-time with an additional identifying
value. This is common when using ASP.NET UserControls or MasterPages to encapsulate UI logic.