So I add the mod to my server and as soon as someone places a preserver this happens:
Null being stored in NBT! / 25.06 02:52:09 [Server] Server thread/ERROR [Sponge]: /*************************************************************************************************************************************/ 25.06 02:52:09 [Server] Server thread/ERROR [Sponge]: / Sponge is forcing a shutdown of the game because someone is storing nulls in an / 25.06 02:52:09 [Server] Server thread/ERROR [Sponge]: / NBTTagCompound. Our implementation and Minecraft's strictly prevents this from / 25.06 02:52:09 [Server] Server thread/ERROR [Sponge]: / happening, however, certain mods are not null checking. Please provide this / 25.06 02:52:09 [Server] Server thread/ERROR [Sponge]: / report in a report to the associated mod! / 25.06 02:52:09 [Server] Server thread/ERROR [Sponge]: / Potential Culprit: erebus / 25.06 02:52:09 [Server] Server thread/ERROR [Sponge]: / NBT Key: EntityNBT / 25.06 02:52:09 [Server] Server thread/ERROR [Sponge]: / Exception! / 25.06 02:52:09 [Server] Server thread/ERROR [Sponge]: / org.spongepowered.api.util.PEBKACException: Someone is trying to store a null to an NBTTagCompound! / 25.06 02:52:09 [Server] Server thread/ERROR [Sponge]: / net.minecraft.nbt.NBTTagCompound.handler$checkNullTag$bcp000(NBTTagCompound.java:1149) / 25.06 02:52:09 [Server] Server thread/ERROR [Sponge]: / net.minecraft.nbt.NBTTagCompound.func_74782_a(NBTTagCompound.java) / 25.06 02:52:09 [Server] Server thread/ERROR [Sponge]: / erebus.tileentity.TileEntityPreservedBlock.func_189515_b(TileEntityPreservedBlock.java:69) / 25.06 02:52:09 [Server] Server thread/ERROR [Sponge]: / net.minecraft.world.WorldServer.createSpongeBlockSnapshot(WorldServer.java:3898) / 25.06 02:52:09 [Server] Server thread/ERROR [Sponge]: / org.spongepowered.common.event.tracking.context.MultiBlockCaptureSupplier.createTransaction(MultiBlockCaptureSupplier.java:588) / 25.06 02:52:09 [Server] Server thread/ERROR [Sponge]: / org.spongepowered.common.event.tracking.TrackingUtil.createTransactionLists(TrackingUtil.java:494) / 25.06 02:52:09 [Server] Server thread/ERROR [Sponge]: / org.spongepowered.common.event.tracking.TrackingUtil.processBlockCaptures(TrackingUtil.java:450) / 25.06 02:52:09 [Server] Server thread/ERROR [Sponge]: / org.spongepowered.common.event.tracking.TrackingUtil.processBlockCaptures(TrackingUtil.java:411) / 25.06 02:52:09 [Server] Server thread/ERROR [Sponge]: / org.spongepowered.common.event.tracking.phase.packet.player.PlaceBlockPacketState.unwind(PlaceBlockPacketState.java:131) / 25.06 02:52:09 [Server] Server thread/ERROR [Sponge]: / org.spongepowered.common.event.tracking.phase.packet.player.PlaceBlockPacketState.unwind(PlaceBlockPacketState.java:68) / 25.06 02:52:09 [Server] Server thread/ERROR [Sponge]: / org.spongepowered.common.event.tracking.PhaseTracker.completePhase(PhaseTracker.java:318) / 25.06 02:52:09 [Server] Server thread/ERROR [Sponge]: / org.spongepowered.common.event.tracking.PhaseContext.close(PhaseContext.java:613) / 25.06 02:52:09 [Server] Server thread/ERROR [Sponge]: / org.spongepowered.common.event.tracking.phase.packet.PacketPhaseUtil.onProcessPacket(PacketPhaseUtil.java:195) / 25.06 02:52:09 [Server] Server thread/ERROR [Sponge]: / net.minecraft.network.PacketThreadUtil$1.redirect$onProcessPacket$zma000(SourceFile:539) / 25.06 02:52:09 [Server] Server thread/ERROR [Sponge]: / net.minecraft.network.PacketThreadUtil$1.run(SourceFile:13) / 25.06 02:52:09 [Server] Server thread/ERROR [Sponge]: / java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) / 25.06 02:52:09 [Server] Server thread/ERROR [Sponge]: / java.util.concurrent.FutureTask.run(FutureTask.java:266) / 25.06 02:52:09 [Server] Server thread/ERROR [Sponge]: / net.minecraft.util.Util.func_181617_a(SourceFile:46) / 25.06 02:52:09 [Server] Server thread/ERROR [Sponge]: / org.spongepowered.common.SpongeImplHooks.onUtilRunTask(SpongeImplHooks.java:307) / 25.06 02:52:09 [Server] Server thread/ERROR [Sponge]: / net.minecraft.server.MinecraftServer.redirect$onRun$zke000(MinecraftServer.java:3970) / 25.06 02:52:09 [Server] Server thread/ERROR [Sponge]: / net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:723) / 25.06 02:52:09 [Server] Server thread/ERROR [Sponge]: / net.minecraft.server.dedicated.DedicatedServer.func_71190_q(DedicatedServer.java:397) / 25.06 02:52:09 [Server] Server thread/ERROR [Sponge]: / net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:668) / 25.06 02:52:09 [Server] Server thread/ERROR [Sponge]: / net.minecraft.server.MinecraftServer.run(MinecraftServer.java:526) / 25.06 02:52:09 [Server] Server thread/ERROR [Sponge]: / java.lang.Thread.run(Thread.java:748) */ 25.06 02:52:09 [Server] Server thread/ERROR [Sponge]:

Seems to be related to #741

The way I understand it, the block, when placed, has EntityNBT set to null. It is then updated a bit later.
It should probably not be set at all initially, only when the actual entity data has arrived.