java.lang.NullPointerException: Rendering screen
at refinedstorage.gui.GuiGrid.drawForeground(GuiGrid.java:136)
at refinedstorage.gui.GuiBase.drawGuiContainerForegroundLayer(GuiBase.java:93)
at net.minecraft.client.gui.inventory.GuiContainer.drawScreen(GuiContainer.java:136)
at net.minecraftforge.client.ForgeHooksClient.drawScreen(ForgeHooksClient.java:313)
at net.minecraft.client.renderer.EntityRenderer.updateCameraAndRender(EntityRenderer.java:1147)
at net.minecraft.client.Minecraft.runGameLoop(Minecraft.java:1135)
at net.minecraft.client.Minecraft.run(Minecraft.java:401)
at net.minecraft.client.main.Main.main(Main.java:118)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at net.minecraft.launchwrapper.Launch.launch(Launch.java:135)
at net.minecraft.launchwrapper.Launch.main(Launch.java:28)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at net.minecraftforge.gradle.GradleStartCommon.launch(GradleStartCommon.java:97)
at GradleStart.main(GradleStart.java:26)

is the items list being modified while reading it.

yes

Is this fixed by your PR's?