Regarding waiting:
http://watirwebdriver.com/waiting/

Watir::Wait.until { ( Code to check Combobox has your value... ) }

Regarding NilClass, the code I gave is assuming that the text field is 
inside a label, like in the gmail page. If this is not the case, you 
will get nil for the parent's label. You need to adapt the code to the 
structure of your page.

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