As per the title, BOP still tries to register disabled (ID=-1) biomes to the Biome Dictionary, triggering an NPE during load.

The easiest fix is to edit addBiomesToDictionary and replace all the calls to BiomeDictionary.registerBiomeType with a method delegate that checks biome != null then passes it on.

EDIT:
I will PR the fix.

EDIT 2: There are multiple other similar issues, including in disableRiver and BOPEntities.registerEntities. The PR has not been updated because I am going to bed. :P

Should still build automatically for 1.7, building is only broken for 1.8.8 at this stage until I get in touch with the Forge guys about issues with the setup. Looks like IronChests and CodeChickenLib are experiencing similar issues. Scroll down on http://files.minecraftforge.net/BiomesOPlenty/ and you should see the new build

Ah, I think I see why the last one didn't come through, you submitted your last PR to the BOP-1.7.10+2-2.0.x branch as opposed to BOP-1.7.10-2.1.x. An understandable mistake to make, admittedly I didn't even notice it myself.

Should be resolved now, a new build should be up in a few.

EDIT: Build's up, changelog is a bit is a bit out of order but it did build from the latest commit.

Hrm, for some reason I thought I never added disabling biome id usage to 1.7, which is probably why some things account for it and others don't. Fortunately this shouldn't be a problem in 1.8, but i'm happy to accept any fixes you want to make to 1.7.

Does BoP build automatically like IC2, or would you have to manually release a new version with the fixes?

My PR did not fix most of the other, similar issues, including the ones mentioned above, so the current release still suffers from them.