'item' cannot be set while loading (Error 378)

You cannot set the specified value while loading. This error has the following cause and solution:

• You tried to modify a property of the Extender object from a UserControl, but the value cannot be changed while the container is loading. For example, you tried to change the Extender.Name property in your ReadProperties event.

  To solve this problem, move your code to an event that occurs later in the load process.