jQuery UI dialog - check if exists by instance method

I'd like to use instance method for testing if jQuery UI Dialog widget has been initialized or not. Regarding to API, this is possible, but it doesn't work for me:

Uncaught Error: cannot call methods on dialog prior to initialization; attempted to call method 'instance'

demo: http://jsfiddle.net/mDbV7/

UPDATE:

This was a mistake in the documentation, instance method will be available from version 1.11.0, see this issue.