I had just put down a neon furnace and got it up and running. Noticed the game started jittering around. Went back to my console log and saw this happening in the background. Perhaps just an unhandled null check for an empty slot?

java.util.concurrent.ExecutionException: java.lang.NullPointerException: The validated object is null
at java.util.concurrent.FutureTask.report(FutureTask.java:122) ~[?:1.8.0_131]
at java.util.concurrent.FutureTask.get(FutureTask.java:192) ~[?:1.8.0_131]
at net.minecraft.util.Util.func_181617_a(SourceFile:47) [h.class:?]
at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:720) [MinecraftServer.class:?]
at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:665) [MinecraftServer.class:?]
at net.minecraft.server.integrated.IntegratedServer.func_71217_p(IntegratedServer.java:239) [chb.class:?]
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:523) [MinecraftServer.class:?]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0_131]
Caused by: java.lang.NullPointerException: The validated object is null
at org.apache.commons.lang3.Validate.notNull(Validate.java:225) ~[commons-lang3-3.5.jar:3.5]
at org.apache.commons.lang3.Validate.notNull(Validate.java:206) ~[commons-lang3-3.5.jar:3.5]
at net.minecraft.util.NonNullList.set(SourceFile:49) ~[fi.class:?]
at futurepack.common.block.machines.TileEntityMachineBase.func_70299_a(TileEntityMachineBase.java:124) ~[TileEntityMachineBase.class:?]
at futurepack.common.gui.GuiEFurnace$ContainerEFurnace.func_82846_b(GuiEFurnace.java:79) ~[GuiEFurnace$ContainerEFurnace.class:?]
at net.minecraft.inventory.Container.func_184996_a(Container.java:244) ~[afp.class:?]
at net.minecraft.network.NetHandlerPlayServer.func_147351_a(NetHandlerPlayServer.java:1148) ~[oy.class:?]
at net.minecraft.network.play.client.CPacketClickWindow.func_148833_a(CPacketClickWindow.java:38) ~[lf.class:?]
at net.minecraft.network.play.client.CPacketClickWindow.func_148833_a(CPacketClickWindow.java:12) ~[lf.class:?]
at net.minecraft.network.PacketThreadUtil$1.run(SourceFile:13) ~[hv$1.class:?]
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) ~[?:1.8.0_131]
at java.util.concurrent.FutureTask.run(FutureTask.java:266) ~[?:1.8.0_131]
at net.minecraft.util.Util.func_181617_a(SourceFile:46) ~[h.class:?]
... 5 more