Issue #3404039 by Wim Leers: PluginExistsConstraintValidator should return early if given NULL

(cherry picked from commit 156533fe)

core/lib/Drupal/Core/Plugin/Plugin/Validation/Constraint/PluginExistsConstraintValidator.php

@@ -20,6 +20,10 @@ class PluginExistsConstraintValidator extends ConstraintValidator {
   public function validate(mixed $plugin_id, Constraint $constraint) {
     assert($constraint instanceof PluginExistsConstraint);
 
+    if ($plugin_id === NULL) {
+      return;
+    }
+
     $definition = $constraint->pluginManager->getDefinition($plugin_id, FALSE);
     // Some plugin managers provide fallbacks.
     if ($constraint->pluginManager instanceof FallbackPluginManagerInterface) {

core/tests/Drupal/KernelTests/Core/Plugin/PluginExistsConstraintValidatorTest.php

@@ -61,6 +61,11 @@ public function testValidation(): void {
     $violations = $data->validate();
     $this->assertCount(1, $violations);
     $this->assertSame("The 'action_test_save_entity' plugin must implement or extend " . MenuInterface::class . '.', (string) $violations->get(0)->getMessage());
+
+    // No validation is attempted on a NULL value.
+    $data->setValue(NULL);
+    $violations = $data->validate();
+    $this->assertCount(0, $violations);
   }
 
 }