Hyperlinks throw assertions for undefined data types

Whilst investigating JBIDE-8064 I ran across a problem with hyperlinks that are supposed to open the WSDL or XSD editor for message or schema definitions. The BPELUtil#openEditor() method will simply throw an assertion if the hyperlink points to an unknown object. This will happen if, for example, the user enters an incorrect message type for a variable using the editor's Source tab, or if a message type has been removed from a WSDL, etc.

The editor should display an error dialog if the object is undefined instead of simply ignoring the user gesture.