PreviewPane: remove redundant null check

Noticed by Findbugs.
merge develop -D org -I include -E exclude: PR 528 (Tile performance improvements for VMS and NDPI)
merge develop -D org -I include -E exclude: PR 635 (Use jgit-buildnumber to set version information in Maven)
merge develop -D org -I include -E exclude: PR 639 (xsd-fu: Extend C++ support and add extra options)
merge develop -D org -I include -E exclude: PR 643 (Data test updates)
merge develop -D org -I include -E exclude: PR 644 (Add support for writing ModuloAlong* annotations)
merge develop -D org -I include -E exclude: PR 645 (Leica LIF: also parse channel names from FilterSettingRecords (rebased onto develop))
merge develop -D org -I include -E exclude: PR 646 (Save current revision number in memo file)
merge develop -D org -I include -E exclude: PR 647 (Make JDK 1.6 the minimum supported version)
merge develop -D org -I include -E exclude: PR 648 (Java 1.7/Findbugs fixes)