Please provide the following information so we can help:
What version of SQL Developer are you using?
What are the results of opening the properties for the connection and doing a test?
Does the navigation tree open correctly?
Are you having trouble with all of your Oracle connections?
When running those queries are they using the same connection as the export?
Provide the details of the NPE when using the Export Wizard.
Exactly what steps are you taking to do the export? (eg, from the Navigator/tools...)
Are there any other errors in the SQL Developer log?
SQL Developer Team