A specific query was copied from other code and was placed in a file. Opened the file in SQL Nav to edit the file, fixing the syntax and removing quotes. When editing the following line SQL Navigator crashed:
sql += " mits_cause_decode mcd, ";
Placed cursor just before 'mits' clicked <backspace> twice to remove the double-quote and SQL Nav simply disappeared.
Editing the same file in v 5.5.4 works fine.
Waiting for fix in a future release of SQL navigator for Oracle