I have tried 2 different versions of steve's carts, the error is repeatable, it appears to be interacting with the better portals mod, the gui opens without a crash as long as a portal isnt open in the loaded chunks.

java.lang.NullPointerException: Rendering screen
at net.minecraft.util.math.AxisAlignedBB.func_72326_a(SourceFile:282)
at de.johni0702.minecraft.betterportals.client.renderer.AbstractRenderPortal$Companion$beforeRender$inPortals$1.apply(AbstractRenderPortal.kt:65)
at de.johni0702.minecraft.betterportals.client.renderer.AbstractRenderPortal$Companion$beforeRender$inPortals$1.apply(AbstractRenderPortal.kt:27)
at net.minecraft.world.World.func_175644_a(World.java:3034)
at de.johni0702.minecraft.betterportals.client.renderer.AbstractRenderPortal$Companion.beforeRender(AbstractRenderPortal.kt:58)
at net.minecraft.client.renderer.entity.RenderManager.handler$beforeRenderEntity$zzi000(RenderManager.java:597)
at net.minecraft.client.renderer.entity.RenderManager.func_188391_a(RenderManager.java)
at vswe.stevescarts.guis.GuiCartAssembler.render3DCart(GuiCartAssembler.java:304)
at vswe.stevescarts.guis.GuiCartAssembler.drawGuiBackground(GuiCartAssembler.java:234)
at vswe.stevescarts.guis.GuiBase.func_146976_a(GuiBase.java:157)
at net.minecraft.client.gui.inventory.GuiContainer.func_73863_a(GuiContainer.java:75)
at vswe.stevescarts.guis.GuiBase.func_73863_a(GuiBase.java:129)
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)

Full error log

crash-2019-07-08_17.35.20-client.txt