Issue #3426324 by Wim Leers: ExistsConstraintValidator should ignore NULL values and treat `core` as a valid module

(cherry picked from commit ab695b5cc6)

core/lib/Drupal/Core/Extension/Plugin/Validation/Constraint/ExtensionExistsConstraintValidator.php

@@ -61,12 +61,26 @@ class ExtensionExistsConstraintValidator extends ConstraintValidator implements
 
     switch ($constraint->type) {
       case 'module':
+        // This constraint may be used to validate nullable (optional) values.
+        if ($extension_name === NULL) {
+          return;
+        }
+        // Some plugins are shipped in `core/lib`, which corresponds to the
+        // special `core` extension name.
+        // For example: \Drupal\Core\Menu\Plugin\Block\LocalActionsBlock.
+        if ($extension_name === 'core') {
+          return;
+        }
         if (!$this->moduleHandler->moduleExists($extension_name)) {
           $this->context->addViolation($constraint->moduleMessage, $variables);
         }
         break;
 
       case 'theme':
+        // This constraint may be used to validate nullable (optional) values.
+        if ($extension_name === NULL) {
+          return;
+        }
         if (!$this->themeHandler->themeExists($extension_name)) {
           $this->context->addViolation($constraint->themeMessage, $variables);
         }

core/tests/Drupal/KernelTests/Core/Extension/ExtensionExistsConstraintValidatorTest.php

@@ -31,8 +31,13 @@ class ExtensionExistsConstraintValidatorTest extends KernelTestBase {
 
     /** @var \Drupal\Core\TypedData\TypedDataManagerInterface $typed_data */
     $typed_data = $this->container->get('typed_data_manager');
-    $data = $typed_data->create($definition, 'user');
 
+    // `core` provides many plugins without the need to install a module.
+    $data = $typed_data->create($definition, 'core');
+    $violations = $data->validate();
+    $this->assertCount(0, $violations);
+
+    $data->setValue('user');
     $violations = $data->validate();
     $this->assertCount(1, $violations);
     $this->assertSame("Module 'user' is not installed.", (string) $violations->get(0)->getMessage());
@@ -40,6 +45,10 @@ class ExtensionExistsConstraintValidatorTest extends KernelTestBase {
     $this->enableModules(['user']);
     $this->assertCount(0, $data->validate());
 
+    // NULL should not trigger a validation error: a value may be nullable.
+    $data->setValue(NULL);
+    $this->assertCount(0, $data->validate());
+
     $definition->setConstraints(['ExtensionExists' => 'theme']);
     $data = $typed_data->create($definition, 'stark');
 
@@ -56,6 +65,17 @@ class ExtensionExistsConstraintValidatorTest extends KernelTestBase {
       ->create($definition, 'stark');
     $this->assertCount(0, $data->validate());
 
+    // `core` provides many plugins without the need to install a module, but it
+    // does not work for themes.
+    $data = $typed_data->create($definition, 'core');
+    $violations = $data->validate();
+    $this->assertCount(1, $violations);
+    $this->assertSame("Theme 'core' is not installed.", (string) $violations->get(0)->getMessage());
+
+    // NULL should not trigger a validation error: a value may be nullable.
+    $data->setValue(NULL);
+    $this->assertCount(0, $data->validate());
+
     // Anything but a module or theme should raise an exception.
     $definition->setConstraints(['ExtensionExists' => 'profile']);
     $this->expectExceptionMessage("Unknown extension type: 'profile'");