This crash has occurred multiple times. I'm working on my own modpack in singleplayer but this event seems to occur randomly whether I have other mods installed or not. I've managed to get the mod to work int he past so I'm not sure what is causing this.

Here is the crash log:
---- Minecraft Crash Report ----
// But it works on my machine.

Time: 12/28/14 6:01 PM
Description: Ticking player

java.lang.NullPointerException: Ticking player
at com.darkona.adventurebackpack.item.ItemAdventureBackpack.onArmorTick(ItemAdventureBackpack.java:215)
at net.minecraft.entity.player.InventoryPlayer.func_70429_k(InventoryPlayer.java:319)
at net.minecraft.entity.player.EntityPlayer.func_70636_d(EntityPlayer.java:538)
at net.minecraft.entity.EntityLivingBase.func_70071_h_(EntityLivingBase.java:1611)
at net.minecraft.entity.player.EntityPlayer.func_70071_h_(EntityPlayer.java:288)
at net.minecraft.entity.player.EntityPlayerMP.func_71127_g(EntityPlayerMP.java:295)
at net.minecraft.network.NetHandlerPlayServer.func_147347_a(NetHandlerPlayServer.java:303)
at net.minecraft.network.play.client.C03PacketPlayer.func_148833_a(SourceFile:137)
at net.minecraft.network.play.client.C03PacketPlayer$C06PacketPlayerPosLook.func_148833_a(SourceFile:20)
at net.minecraft.network.NetworkManager.func_74428_b(NetworkManager.java:212)
at net.minecraft.network.NetworkSystem.func_151269_c(NetworkSystem.java:165)
at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:659)
at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:547)
at net.minecraft.server.integrated.IntegratedServer.func_71217_p(IntegratedServer.java:111)
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:427)
at net.minecraft.server.MinecraftServer$2.run(MinecraftServer.java:685)

A detailed walkthrough of the error, its code path and all known details is as follows:

-- Head --
Stacktrace:
at com.darkona.adventurebackpack.item.ItemAdventureBackpack.onArmorTick(ItemAdventureBackpack.java:215)
at net.minecraft.entity.player.InventoryPlayer.func_70429_k(InventoryPlayer.java:319)
at net.minecraft.entity.player.EntityPlayer.func_70636_d(EntityPlayer.java:538)
at net.minecraft.entity.EntityLivingBase.func_70071_h_(EntityLivingBase.java:1611)
at net.minecraft.entity.player.EntityPlayer.func_70071_h_(EntityPlayer.java:288)

-- Player being ticked --
Details:
Entity Type: null (net.minecraft.entity.player.EntityPlayerMP)
Entity ID: 336
Entity Name: Renloris
Entity's Exact location: 152.48, 64.00, 290.85
Entity's Block location: World: (152,64,290), Chunk: (at 8,4,2 in 9,18; contains blocks 144,0,288 to 159,255,303), Region: (0,0; contains chunks 0,0 to 31,31, blocks 0,0,0 to 511,255,511)
Entity's Momentum: 0.00, -0.08, 0.00
Stacktrace:
at net.minecraft.entity.player.EntityPlayerMP.func_71127_g(EntityPlayerMP.java:295)
at net.minecraft.network.NetHandlerPlayServer.func_147347_a(NetHandlerPlayServer.java:303)
at net.minecraft.network.play.client.C03PacketPlayer.func_148833_a(SourceFile:137)
at net.minecraft.network.play.client.C03PacketPlayer$C06PacketPlayerPosLook.func_148833_a(SourceFile:20)
at net.minecraft.network.NetworkManager.func_74428_b(NetworkManager.java:212)

-- Ticking connection --
Details:
Connection: net.minecraft.network.NetworkManager@5108bba6
Stacktrace:
at net.minecraft.network.NetworkSystem.func_151269_c(NetworkSystem.java:165)
at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:659)
at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:547)
at net.minecraft.server.integrated.IntegratedServer.func_71217_p(IntegratedServer.java:111)
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:427)
at net.minecraft.server.MinecraftServer$2.run(MinecraftServer.java:685)
Sidenote: Whenever this issues occurs, the value of liquid in the backpack tanks is not shown on the item before you put it on.

Fixed in 0.7.4