These issues were all found while trying to use the "Calculate Previews"
feature of the export wizard.
gen/proxy/referencedbyselection.py: ProxyDbBase need not define the
writable functions (add_*, commit_*, del_*) in its __tables. ProxyDbBase
doesn't have methods for add_person, etc, so this was causing an
AttributeError
gen/proxy/referencedbyselection.py: Add some handle checks to prevent
lookups on None handles
gen/proxy/proxybase.py: remove redundant test