Fix another structure generator NPE

Spigot-Server-Patches/0054-Generator-Settings.patch

@@ -1,4 +1,4 @@
-From c903fee4eac76019e6961b8f046a5dc93fe96a3b Mon Sep 17 00:00:00 2001
+From aac3585edb181f425933b7e2034f351f64aa3b62 Mon Sep 17 00:00:00 2001
 From: Byteflux <byte@byteflux.net>
 Date: Tue, 2 Jun 2015 00:41:23 -0700
 Subject: [PATCH] Generator Settings
@@ -223,7 +223,7 @@ index 2f01bbf..0156892 100644
  
      public Chunk getChunkAt(BlockPosition blockposition) {
 diff --git a/src/main/java/net/minecraft/server/StructureGenerator.java b/src/main/java/net/minecraft/server/StructureGenerator.java
-index 0f8211a..859fcfc 100644
+index 0f8211a..e76acfc 100644
 --- a/src/main/java/net/minecraft/server/StructureGenerator.java
 +++ b/src/main/java/net/minecraft/server/StructureGenerator.java
 @@ -88,6 +88,7 @@ public abstract class StructureGenerator extends WorldGenBase {
@@ -234,6 +234,14 @@ index 0f8211a..859fcfc 100644
          this.a(this.c);
          return this.c(blockposition) != null;
      }
+@@ -115,6 +116,7 @@ public abstract class StructureGenerator extends WorldGenBase {
+     }
+ 
+     public boolean a(World world, BlockPosition blockposition) {
++        if (this.c == null) return false; // PaperSpigot
+         this.a(world);
+         Iterator iterator = this.e.values().iterator();
+ 
 diff --git a/src/main/java/org/github/paperspigot/PaperSpigotWorldConfig.java b/src/main/java/org/github/paperspigot/PaperSpigotWorldConfig.java
 index 557561c..8491d3b 100644
 --- a/src/main/java/org/github/paperspigot/PaperSpigotWorldConfig.java