I am triying to change the recipe of the backpacks with crafttweaker. To keep the inventory I use

recipes.addShaped
("customDBagT2",
forestry:digger_bag_t2,
[[null,forestry:digger_bag.marked("bag"), null],[minecraft:string,ironchest:iron_chest:2,minecraft:string],[minecraft:wool:*,minecraft:wool:*,minecraft:wool:*]],
function(output, inputs, crafting) {
return forestry:digger_bag_t2.withTag(inputs.bag.tag);
},
null);

It works fine at normal worktables, but the forestry crafttable spams
ERROR: Could not execute RecipeFuntion
everytime you open the UI after the first craft.

I considered long and hard whether to post this here or at CraftTweaker... decided for here since opening JEI doesnt produce this error and I dont see, why the crafttable would try to call the function, before it tries to actually craft it. Sorry, if that's wrong :) Thnx for an awesome mod!

P.S.: Tried adding a conditional statement to check if theres is a NBT tag, didnt help either.

Just found that there was not only the in game error but also sth in the CraftTweaker log... Now I'm double guessing myelf again, if I sould have posted at the CraftTweaker GitHub, although I count 7 "forestry" and only 3 "crafttweaker", that must be a good measure, right? :s

java.lang.NullPointerException
at ZenClassCrafttweakerForestry4.process(Unknown Source)
at crafttweaker.mc1120.recipes.MCRecipeShaped.getCraftingResult(MCRecipeShaped.java:109)
at crafttweaker.mc1120.recipes.MCRecipeShaped.func_77572_b(MCRecipeShaped.java:73)
at forestry.worktable.recipes.MemorizedRecipe.getOutputIcon(MemorizedRecipe.java:93)
at forestry.worktable.recipes.RecipeMemory.getRecipeDisplayOutput(RecipeMemory.java:127)
at forestry.worktable.gui.widgets.MemorizedRecipeSlot.getItemStack(MemorizedRecipeSlot.java:37)
at forestry.core.gui.widgets.ItemStackWidgetBase.getToolTip(ItemStackWidgetBase.java:43)
at forestry.core.gui.GuiUtil.drawToolTips(GuiUtil.java:79)
at forestry.core.gui.GuiForestry.func_146979_b(GuiForestry.java:234)
at net.minecraft.client.gui.inventory.GuiContainer.func_73863_a(GuiContainer.java:117)
at forestry.core.gui.GuiForestry.func_73863_a(GuiForestry.java:109)
at net.minecraftforge.client.ForgeHooksClient.drawScreen(ForgeHooksClient.java:381)
at net.minecraft.client.renderer.EntityRenderer.func_181560_a(EntityRenderer.java:1124)
at net.minecraft.client.Minecraft.func_71411_J(Minecraft.java:1118)
at net.minecraft.client.Minecraft.func_99999_d(Minecraft.java:397)
at net.minecraft.client.main.Main.main(SourceFile:123)
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:497)
at net.minecraft.launchwrapper.Launch.launch(Launch.java:135)
at net.minecraft.launchwrapper.Launch.main(Launch.java:28)