JavaScript Alert: The Panel specified in ResizePanel1ID cannot be located

Symptoms

Using a PanelSet.Splitter with resizing capability, a JavaScript Alert appears with details stating a specified Panel cannot be located.

Cause

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.

Resolution/Workaround

When this occurs, it is best to set the value of the ResizePanel1ID and ResizePanel2ID properties programmatically with a Panel's ClientID, which will return the correct run-time value:

Splitter.ResizePanel1ID = Panel.ClientID;