Fix possible NPE

git-svn-id: svn+ssh://svn.code.sf.net/p/pcgen/code/Trunk@21587 ef53c05b-3124-4d59-ae7d-fe3273f42be7
Allow new UI tabs to be hidden using the GAMEMODE Tab token
* Full hiding implemented
* Also corrected an issue where todo links weren't swapping to renamed tabs.
Issue#: CODE-2027


git-svn-id: svn+ssh://svn.code.sf.net/p/pcgen/code/Trunk@21588 ef53c05b-3124-4d59-ae7d-fe3273f42be7