Tinyumbrella should be updated this weekend as Notcom tweeted. In the mean time you can use Redsn0w (iPhone dev team) to manually grab and upload SHSH to cydia. Follow the tutorial here:
Plus, if you previously checked Make my life easier when Cydia offered it, then your 6.1.2 SHSH are already saved.
Check the Cydia front page (at the top) to verify.