Biome: Added UnknownBiome class to avoid astonishing behaviour on unknown biomes

src/pocketmine/level/biome/Biome.php

@@ -107,7 +107,10 @@ abstract class Biome{
 	 * @return Biome
 	 */
 	public static function getBiome(int $id) : Biome{
-		return self::$biomes[$id] ?? self::$biomes[self::OCEAN];
+		if(self::$biomes[$id] === null){
+			self::register($id, new UnknownBiome());
+		}
+		return self::$biomes[$id];
 	}
 
 	public function clearPopulators(){

src/pocketmine/level/biome/UnknownBiome.php

@@ -0,0 +1,34 @@
+<?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\level\biome;
+
+/**
+ * Polyfill class for biomes that are unknown to PocketMine-MP
+ */
+class UnknownBiome extends Biome{
+
+	public function getName() : string{
+		return "Unknown";
+	}
+}