Fix MC-194811 - Removing structures causes chunk save errors.

patches/minecraft/net/minecraft/world/chunk/storage/ChunkSerializer.java.patch

@@ -17,6 +17,15 @@
                    chunkprimer.func_201637_h(blockpos);
                 }
              }
+@@ -167,7 +168,7 @@
+       Heightmap.func_222690_a(ichunk, enumset);
+       CompoundNBT compoundnbt4 = compoundnbt.func_74775_l("Structures");
+       ichunk.func_201612_a(func_235967_a_(p_222656_1_, compoundnbt4, p_222656_0_.func_72905_C()));
+-      ichunk.func_201606_b(func_227075_a_(p_222656_3_, compoundnbt4));
++      net.minecraftforge.common.ForgeHooks.fixNullStructureReferences(ichunk, func_227075_a_(p_222656_3_, compoundnbt4));
+       if (compoundnbt.func_74767_n("shouldSave")) {
+          ichunk.func_177427_f(true);
+       }
 @@ -183,6 +184,7 @@
        }
  

src/main/java/net/minecraftforge/common/ForgeHooks.java

@@ -43,6 +43,8 @@ import com.mojang.serialization.codecs.RecordCodecBuilder;
 
 import javax.annotation.Nonnull;
 import javax.annotation.Nullable;
+
+import it.unimi.dsi.fastutil.longs.LongSet;
 import net.minecraft.advancements.Advancement;
 import net.minecraft.block.Block;
 import net.minecraft.block.material.Material;
@@ -91,6 +93,8 @@ import net.minecraft.potion.PotionUtils;
 import net.minecraft.stats.Stats;
 import net.minecraft.util.math.vector.Vector3d;
 import net.minecraft.util.text.*;
+import net.minecraft.world.chunk.IChunk;
+import net.minecraft.world.gen.feature.structure.Structure;
 import net.minecraft.world.spawner.AbstractSpawner;
 import net.minecraft.tileentity.FurnaceTileEntity;
 import net.minecraft.tileentity.TileEntity;
@@ -1232,4 +1236,18 @@ public class ForgeHooks
         modpacks.add("vanilla");
         return modpacks;
     }
+
+    /**
+     * Fixes MC-194811
+     * When a structure mod is removed, this map may contain null keys. This will make the world unable to save if this persists.
+     * If we remove a structure from the save data in this way, we then mark the chunk for saving
+     */
+    public static void fixNullStructureReferences(IChunk chunk, Map<Structure<?>, LongSet> structureReferences)
+    {
+        if (structureReferences.remove(null) != null)
+        {
+            chunk.setModified(true);
+        }
+        chunk.setStructureReferences(structureReferences);
+    }
 }