Issue Description:
Game crash.

What happens:
Pick up a slashblade mod item when a backpack upgrade with filterModSpecificUpgrade and set up with any item.

What you expected to happen:
Normally work with SlashBlade mod，or at least do not cause crash.

Steps to reproduce (important):

1. Add both slashblade mod and iron backpack mod to minecraft.
2. Upgrade a backpack with filterModSpecificUpgrade.
3. Set up any item for the backpack to pick up.
4. Pick up any slashblade mod item(such as proudsoul).
5. The item will still on the ground actually,but the game crashes.

Affected Versions (Do not use "latest"):

• IronBackpacks: 1.1.2
• Minecraft: 1.7.10
• Forge: 10.13.4.1664
• Other Conflicting Mods (may be irrelevant to your issue):SlashBlade version:r88