fix wrong return type of LoggerHandler::getInstance

src/LoggerHandler.php

@@ -20,10 +20,14 @@ use Psr\Log\NullLogger;
 
 class LoggerHandler
 {
+    /** @var LoggerHandler */
     private static $instance = null;
     private $logger;
 
-    public static function getInstance(): ?LoggerHandler
+    /**
+     * @return LoggerHandler
+     */
+    public static function getInstance(): LoggerHandler
     {
         if (self::$instance === null) {
             self::$instance = new self();