Bug FTB Beyond - Shift+R Click in Iron Backpacks sometimes causes crashes

[Gabeux]

Hi there!

Sometimes when Shift+R clicking an Iron Backpack (from any tier) will cause a client crash. I'm not sure what conditions cause the crashes, as sometimes it works correctly, and sometimes it just breaks.

Here's the log:

java.lang.NullPointerException: Rendering screen
at net.minecraft.client.gui.inventory.GuiContainer.func_73863_a(GuiContainer.java:169)
at gr8pefish.ironbackpacks.client.gui.inventory.GUIBackpackAlternate.func_73863_a(GUIBackpackAlternate.java:532)
at net.minecraftforge.client.ForgeHooksClient.drawScreen(ForgeHooksClient.java:382)
at net.minecraft.client.renderer.EntityRenderer.func_181560_a(EntityRenderer.java:1094)
at net.minecraft.client.Minecraft.func_71411_J(Minecraft.java:1076)
at net.minecraft.client.Minecraft.func_99999_d(Minecraft.java:371)
at net.minecraft.client.main.Main.main(SourceFile:124)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:483)
at net.minecraft.launchwrapper.Launch.launch(Launch.java:135)
at net.minecraft.launchwrapper.Launch.main(Launch.java:28)

Thanks!

E: Happening in Beyond 1.3.0 Beta, happened on 1.2.1 too.