fix: exception instead of php warning

Author: Daemant

src/Exception/NotCallableResourceException.php

@@ -0,0 +1,20 @@
+<?php
+
+/*****************************************************************************
+ *
+ *  PROJECT:     MTA PHP SDK
+ *  LICENSE:     See LICENSE in the top level directory
+ *  FILE:        NotCallableResourceException.php
+ *
+ *  Multi Theft Auto is available from http://www.multitheftauto.com/
+ *
+ *****************************************************************************/
+
+declare(strict_types=1);
+
+namespace MultiTheftAuto\Sdk\Exception;
+
+class NotCallableResourceException extends MessageException
+{
+    protected const EXCEPTION_MESSAGE = 'There was a problem with the request. Ensure that the resource handling the call is running.';
+}

src/Transformer/ElementTransformer.php

@@ -15,6 +15,7 @@
 namespace MultiTheftAuto\Sdk\Transformer;
 
 use MultiTheftAuto\Sdk\Factory\ElementFactory;
+use MultiTheftAuto\Sdk\Exception\NotCallableResourceException;
 
 abstract class ElementTransformer
 {
@@ -29,8 +30,12 @@ public static function fromServer(?string $dataFromServer): ?array
 
         $data = json_decode($dataFromServer);
 
-        foreach ($data as &$value) {
-            ElementTransformer::stringValuesToObjects($value);
+        if ($data !== NULL) {
+            foreach ($data as &$value) {
+                ElementTransformer::stringValuesToObjects($value);
+            }
+        } else {
+            throw new NotCallableResourceException();
         }
 
         return $data;