Property :: validate - Check whether property handle is valid

Usually a property handle is valid when it has been opened successfully. When one of the upper handles, however, is weak-typed or untyped the handle may become invalid when changing the selection in the upper node. The function resolves the problem.

Usually, the function is called internally whenever required and need not to be called explicitly.

The function throws an error, when the property handle is not valid or cannot be reopened.