generator: prevent access to chunks that don't exist on this thread

the generator shouldn't be creating chunks that it wasn't given.

src/pocketmine/world/generator/GeneratorChunkManager.php

@@ -0,0 +1,38 @@
+<?php
+
+/*
+ *
+ *  ____            _        _   __  __ _                  __  __ ____
+ * |  _ \ ___   ___| | _____| |_|  \/  (_)_ __   ___      |  \/  |  _ \
+ * | |_) / _ \ / __| |/ / _ \ __| |\/| | | '_ \ / _ \_____| |\/| | |_) |
+ * |  __/ (_) | (__|   <  __/ |_| |  | | | | | |  __/_____| |  | |  __/
+ * |_|   \___/ \___|_|\_\___|\__|_|  |_|_|_| |_|\___|     |_|  |_|_|
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * @author PocketMine Team
+ * @link http://www.pocketmine.net/
+ *
+ *
+*/
+
+declare(strict_types=1);
+
+namespace pocketmine\world\generator;
+
+use pocketmine\world\format\Chunk;
+use pocketmine\world\SimpleChunkManager;
+use pocketmine\world\World;
+
+class GeneratorChunkManager extends SimpleChunkManager{
+
+	public function getChunk(int $chunkX, int $chunkZ, bool $create = false) : ?Chunk{
+		if(!isset($this->chunks[World::chunkHash($chunkX, $chunkZ)])){
+			throw new \InvalidArgumentException("Chunk does not exist");
+		}
+		return parent::getChunk($chunkX, $chunkZ, $create);
+	}
+}

src/pocketmine/world/generator/GeneratorRegisterTask.php

@@ -26,7 +26,6 @@ namespace pocketmine\world\generator;
 use pocketmine\block\BlockFactory;
 use pocketmine\scheduler\AsyncTask;
 use pocketmine\world\biome\Biome;
-use pocketmine\world\SimpleChunkManager;
 use pocketmine\world\World;
 use function serialize;
 use function unserialize;
@@ -50,7 +49,7 @@ class GeneratorRegisterTask extends AsyncTask{
 	public function onRun() : void{
 		BlockFactory::init();
 		Biome::init();
-		$manager = new SimpleChunkManager($this->worldHeight);
+		$manager = new GeneratorChunkManager($this->worldHeight);
 		$this->worker->saveToThreadStore("generation.world{$this->worldId}.manager", $manager);
 
 		/**