Small bodgey patch to fix NPEs when turtles place tiles

src/main/java/dan200/computercraft/shared/mixin/MixinWorld.java

@@ -0,0 +1,55 @@
+/*
+ * This file is part of ComputerCraft - http://www.computercraft.info
+ * Copyright Daniel Ratcliffe, 2011-2019. Do not distribute without permission.
+ * Send enquiries to dratcliffe@gmail.com
+ */
+
+package dan200.computercraft.shared.mixin;
+
+import dan200.computercraft.shared.common.TileGeneric;
+import net.minecraft.block.entity.BlockEntity;
+import net.minecraft.util.math.BlockPos;
+import net.minecraft.world.World;
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.Shadow;
+import org.spongepowered.asm.mixin.injection.At;
+import org.spongepowered.asm.mixin.injection.Inject;
+import org.spongepowered.asm.mixin.injection.callback.CallbackInfo;
+
+import javax.annotation.Nullable;
+import java.util.Collection;
+
+/**
+ * Horrible bodge to ensure a {@link BlockEntity}'s world is always present when setting a TE during another TE's tick.
+ *
+ * Forge does this, this is just a bodge to get Fabric in line with that behaviour.
+ */
+@Mixin( World.class )
+public class MixinWorld
+{
+    @Shadow
+    protected boolean iteratingTickingBlockEntities;
+
+    @Inject( method = "setBlockEntity", at = @At( "HEAD" ) )
+    public void setBlockEntity( BlockPos pos, @Nullable BlockEntity entity, CallbackInfo info )
+    {
+        if( !World.isHeightInvalid( pos ) && entity != null && !entity.isInvalid() && iteratingTickingBlockEntities )
+        {
+            setWorld( entity, this );
+        }
+    }
+
+    @Inject( method = "addBlockEntities", at = @At( "HEAD" ) )
+    public void addBlockEntities( Collection<BlockEntity> entities, CallbackInfo info )
+    {
+        if( iteratingTickingBlockEntities )
+        {
+            for( BlockEntity entity : entities ) setWorld( entity, this );
+        }
+    }
+
+    private static void setWorld( BlockEntity entity, Object world )
+    {
+        if( entity.getWorld() != world && entity instanceof TileGeneric ) entity.setWorld( (World) world );
+    }
+}

src/main/resources/computercraft.common.json

@@ -5,7 +5,8 @@
     "mixins": [
         "MixinBlock",
         "MixinEntity",
-        "MixinServerWorld"
+        "MixinServerWorld",
+        "MixinWorld"
     ],
     "injectors": {
         "defaultRequire": 1