MAGETWO-88187: Magento 2.1.3 to 2.2.1 upgrade issue

Author: svitja

app/code/Magento/Theme/Model/Config/Processor/DesignTheme.php

@@ -72,8 +72,8 @@ public function process(array $config)
     private function changeThemeFullPathToIdentifier($configItems)
     {
         $theme = null;
-        if ($this->arrayManager->exists(DesignInterface::XML_PATH_THEME_ID, $configItems)) {
-            $themeIdentifier = $this->arrayManager->get(DesignInterface::XML_PATH_THEME_ID, $configItems);
+        $themeIdentifier = $this->arrayManager->get(DesignInterface::XML_PATH_THEME_ID, $configItems);
+        if (!empty($themeIdentifier)) {
             if (!is_numeric($themeIdentifier)) {
                 // workaround for case when db is not available
                 try {

app/code/Magento/Theme/Test/Unit/Model/Config/Processor/DesignThemeTest.php

@@ -5,7 +5,6 @@
  */
 namespace Magento\Theme\Test\Unit\Model\Config\Processor;
 
-use Magento\Config\App\Config\Source\DumpConfigSourceAggregated;
 use Magento\Framework\Stdlib\ArrayManager;
 use Magento\Framework\View\Design\Theme\ListInterface;
 use Magento\Theme\Model\Config\Processor\DesignTheme;
@@ -162,6 +161,22 @@ public function getDumpConfigDataProvider()
                         ],
                     ],
                 ],
+                [
+                    [
+                        'websites' => [
+                            'base' => [
+                                'design' => ['theme' => ['theme_id' => '']],
+                            ],
+                        ],
+                    ],
+                    [
+                        'websites' => [
+                            'base' => [
+                                'design' => ['theme' => ['theme_id' => '']],
+                            ],
+                        ],
+                    ],
+                ],
             ],
         ];
     }

dev/tests/integration/testsuite/Magento/Theme/Model/Config/Processor/DesignThemeTest.php

@@ -0,0 +1,27 @@
+<?php
+/**
+ * Copyright © Magento, Inc. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+namespace Magento\Theme\Model\Config\Processor;
+
+use Magento\TestFramework\Helper\Bootstrap;
+
+class DesignThemeTest extends \PHPUnit\Framework\TestCase
+{
+    /**
+     * Verifies that empty 'design/theme/theme_id' config value is processed without errors.
+     */
+    public function testProcessWithEmptyThemeId()
+    {
+        $designTheme = Bootstrap::getObjectManager()->create(DesignTheme::class);
+
+        $config = [
+            'default' => [
+                'design' => ['theme' => ['theme_id' => '']],
+            ],
+        ];
+
+        $this->assertEquals($config, $designTheme->process($config));
+    }
+}