Issue #2594937 by Peacog, cilefen, Sagar Ramgade, chgasparoto, aerozeppelin,...

Issue #2594937 by Peacog, cilefen, Sagar Ramgade, chgasparoto, aerozeppelin, iStryker, feraline, alvar0hurtad0, Cottser, Wim Leers, xjm, sebas5384, almaudoh, gvso, racarvalho1987: Empty 'libraries:' in theme .info.yml file produces confusing PHP warning

core/lib/Drupal/Core/Extension/ThemeHandler.php

@@ -198,8 +198,10 @@ public function listInfo() {
    * {@inheritdoc}
    */
   public function addTheme(Extension $theme) {
-    foreach ($theme->info['libraries'] as $library => $name) {
-      $theme->libraries[$library] = $name;
+    if (!empty($theme->info['libraries'])) {
+      foreach ($theme->info['libraries'] as $library => $name) {
+        $theme->libraries[$library] = $name;
+      }
     }
     if (isset($theme->info['engine'])) {
       $theme->engine = $theme->info['engine'];

core/modules/system/tests/themes/test_theme_libraries_empty/test_theme_libraries_empty.info.yml

+name: 'Test theme libraries empty'
+type: theme
+description: 'Test theme with empty libraries in theme.info.yml'
+version: VERSION
+base theme: classy
+core: 8.x
+libraries:
+

core/tests/Drupal/Tests/Core/Extension/ThemeHandlerTest.php

@@ -137,6 +137,20 @@ public function testRebuildThemeData() {
     $this->assertEquals(array('seven/global-styling'), $info->info['libraries']);
   }
 
+  /**
+   * Tests empty libraries in theme.info.yml file.
+   */
+  public function testThemeLibrariesEmpty() {
+    $theme = new Extension($this->root, 'theme', '/core/modules/system/tests/themes/test_theme_libraries_empty', 'test_theme_libraries_empty.info.yml');
+    try {
+      $this->themeHandler->addTheme($theme);
+      $this->assertTrue(TRUE, 'Empty libraries key in theme.info.yml does not cause PHP warning');
+    }
+    catch (\Exception $e) {
+      $this->fail('Empty libraries key in theme.info.yml causes PHP warning.');
+    }
+  }
+
   /**
    * Tests rebuild the theme data with theme parents.
    */