I faced the same exception while trying to test my web-services deployed to WSO2 ESB.
WSO2 generated both wsdl and wsdl2. I tried to pass a wsdl2 URL and got the above exception. Quick googling showed me, that one of differences between wsdl1.1 and wsdl2.0 is replacing 'definitions' element with 'description'. Also, I found out, that SoapUI does not support wsdl2.
Therefore, for me the solution was to use wsdl1 url instead of wsdl2.