Clean up default level provider management

src/pocketmine/Server.php

@@ -1080,12 +1080,7 @@ class Server{
 			$generator = GeneratorManager::getGenerator($this->getLevelType());
 		}
 
-		if(($providerClass = LevelProviderManager::getProviderByName($this->getProperty("level-settings.default-format", "pmanvil"))) === null){
+		$providerClass = LevelProviderManager::getDefault();
-			$providerClass = LevelProviderManager::getProviderByName("pmanvil");
-			if($providerClass === null){
-				throw new \InvalidStateException("Default level provider has not been registered");
-			}
-		}
 
 		$path = $this->getDataPath() . "worlds/" . $name . "/";
 		/** @var LevelProvider $providerClass */
@@ -1677,6 +1672,12 @@ class Server{
 			$this->network->registerInterface(new RakLibInterface($this));
 
 			LevelProviderManager::init();
+			if(($format = LevelProviderManager::getProviderByName($formatName = (string) $this->getProperty("level-settings.default-format"))) !== null){
+				LevelProviderManager::setDefault($format);
+			}elseif($formatName !== ""){
+				$this->logger->warning($this->language->translateString("pocketmine.level.badDefaultFormat", [$formatName]));
+			}
+
 			if(extension_loaded("leveldb")){
 				$this->logger->debug($this->getLanguage()->translateString("pocketmine.debug.enable"));
 			}

src/pocketmine/level/format/io/LevelProviderManager.php

@@ -32,6 +32,9 @@ use pocketmine\utils\Utils;
 abstract class LevelProviderManager{
 	protected static $providers = [];
 
+	/** @var string|LevelProvider */
+	private static $default = PMAnvil::class;
+
 	public static function init() : void{
 		self::addProvider(Anvil::class);
 		self::addProvider(McRegion::class);
@@ -39,6 +42,29 @@ abstract class LevelProviderManager{
 		self::addProvider(LevelDB::class);
 	}
 
+	/**
+	 * Returns the default format used to generate new levels.
+	 *
+	 * @return string
+	 */
+	public static function getDefault() : string{
+		return self::$default;
+	}
+
+	/**
+	 * Sets the default format.
+	 *
+	 * @param string $class Class extending LevelProvider
+	 *
+	 * @throws \InvalidArgumentException
+	 */
+	public static function setDefault(string $class) : void{
+		Utils::testValidInstance($class, LevelProvider::class);
+
+		self::addProvider($class);
+		self::$default = $class;
+	}
+
 	/**
 	 * @param string $class
 	 *