Avoid WSOD when adding a new gateway (#606)

Avoid WSOD when adding a new gateway when no plugins are available

Author: mglaman

modules/payment/src/Form/PaymentGatewayForm.php

@@ -36,6 +36,20 @@ public static function create(ContainerInterface $container) {
     );
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function buildForm(array $form, FormStateInterface $form_state) {
+    if (empty($this->pluginManager->getDefinitions())) {
+      $form['warning'] = [
+        '#markup' => $this->t('No payment gateway plugins found. Please install a module which provides one.'),
+      ];
+      return $form;
+    }
+
+    return parent::buildForm($form, $form_state);
+  }
+
   /**
    * {@inheritdoc}
    */