bugfix: getClassName function fail if rootElement is not defined

[cstoquer]

Got this problem: If a non initialised wuiDom try to call getClassNames then an exception is thrown.

Fix: we check if rootElement exist, and if not, getClassNames will return an empty array (non initialised wuiDom have no class)

[ronkorving]

You would probably have to add this test to a dozen other APIs then too.
Now, I don't like exceptions of the "foo of undefined" style. But trying to set a className on something that doesn't exist is either:

• a bad idea -> give me a useful error, don't ignore the call silently.
• an early "setup" of DOM that is going to be initialized (in which case, the moment you assign DOM it should get that class name).

What do you think?

[cstoquer]

+1 for the first point.
i figured out what was the source of the problem.
closing this PR.

[ronkorving]

Apologies for confusing your request with setClassNames by the way.

[micky2be]

Not sure if it's related to your problem but we should remove #assign all together.
And create an error on the WuiDom instantiation if the first argument is missing.

A near future version will probably output warnings for the #assign users.