PreviewPane: remove redundant null check

Noticed by Findbugs.
merge dev_4_4 -D org -I include -E exclude: PR 622 (Upgrade SLF4J to 1.7.2 [dev_4_4])
merge dev_4_4 -D org -I include -E exclude: PR 650 (Java 1.7/Findbugs fixes (rebased onto dev_4_4))