I got this error several times and the game crashed.

java.lang.NullPointerException: Tessellating CCL block in world
at net.minecraftforge.client.model.pipeline.BlockInfo.updateLightMatrix(BlockInfo.java:124)
at net.minecraftforge.client.model.pipeline.VertexLighterSmoothAo.updateBlockInfo(VertexLighterSmoothAo.java:186)
at cofh.thermaldynamics.duct.attachments.cover.CoverRenderer.renderBlockQuads(CoverRenderer.java:69)
at cofh.thermaldynamics.duct.attachments.cover.CoverRenderer.renderBlockCover(CoverRenderer.java:140)
at cofh.thermaldynamics.duct.attachments.cover.Cover.render(Cover.java:119)
at cofh.thermaldynamics.render.RenderDuct.renderBlock(RenderDuct.java:387)
at codechicken.lib.render.block.BlockRenderingRegistry.renderBlock(BlockRenderingRegistry.java:94)
at codechicken.lib.render.block.CCBlockRendererDispatcher.func_175018_a(CCBlockRendererDispatcher.java:53)
at mods.betterfoliage.client.Hooks.renderWorldBlock(Hooks.kt:95)
at net.minecraft.client.renderer.chunk.RenderChunk.func_178581_b(RenderChunk.java:293)
at net.minecraft.client.renderer.chunk.ChunkRenderWorker.func_178474_a(ChunkRenderWorker.java:119)
at net.minecraft.client.renderer.chunk.ChunkRenderWorker.run(ChunkRenderWorker.java:47)
at java.lang.Thread.run(Unknown Source)

Full log, and this is not the place for this issue but whatever. Full log, no smug reports.

Sorry, I came to the false page. But here is the file.
crash-2017-08-22_09.13.04-client.txt

What type of cover did you place on the duct? anything from chisel?

Chisel is not present. There are dirt covers and covers made from black granite cobblestone of Underground Biomes Contructs.

Ok, CoFH issue reopened, its valid.