Added a check for Simple Search block to work only if "Enable Search Fields" is enable.

src/Form/SearchForm.php

@@ -23,6 +23,17 @@ class SearchForm  extends FormBase
    */
   public function buildForm(array $form, FormStateInterface $form_state)
   {
+    $config = \Drupal::config(SettingsForm::CONFIG_NAME);
+
+    if (!$config->get(SettingsForm::SEARCH_ALL_FIELDS_FLAG)) { 
+      $form['search-attributes'][SettingsForm::SEARCH_ALL_FIELDS_FLAG] = [
+        '#markup' => $this
+          ->t('<strong>This block is required to enable searching all fields for the Advanced Search.
+            To proceed, please enable the Search All fields in 
+            <a href="/admin/config/search/advanced" target="_blank">Advanced Seach Configuration</a></strong>.'),
+      ];
+    }
+    else {
       $block = \Drupal\block\Entity\Block::load("search");
 
       if ($block) {
@@ -42,6 +53,8 @@ class SearchForm  extends FormBase
         '#value' => (!empty($settings['search_submit_label']) ? $settings['search_submit_label'] : 'Search'),
         '#button_type' => 'primary',
       );
+    }
+    
     return $form;
   }
 

src/Plugin/Block/SearchBlock.php

@@ -4,6 +4,7 @@ namespace Drupal\advanced_search\Plugin\Block;
 
 use Drupal\Core\Block\BlockBase;
 use Drupal\Core\Form\FormStateInterface;
+use Drupal\advanced_search\Form\SettingsForm;
 
 /**
  * Provides a 'SearchBlock' block.
@@ -27,10 +28,21 @@ class SearchBlock extends BlockBase {
    * {@inheritdoc}
    */
   public function blockForm($form, FormStateInterface $form_state) {
+    $config = \Drupal::config(SettingsForm::CONFIG_NAME);
     $form['search-attributes'] = [
       '#type' => 'fieldset',
       '#title' => $this->t('Configure Search Block'),
     ];
+
+    if (!$config->get(SettingsForm::SEARCH_ALL_FIELDS_FLAG)) { 
+      $form['search-attributes'][SettingsForm::SEARCH_ALL_FIELDS_FLAG] = [
+        '#markup' => $this
+          ->t('<strong>This block is required to enable searching all fields for the Advanced Search.
+            To proceed, please enable the Search All fields in 
+            <a href="/admin/config/search/advanced" target="_blank">Advanced Seach Configuration</a></strong>.'),
+      ];
+    }
+    else {
       $views = \Drupal::EntityTypeManager()->getStorage('view')->loadMultiple();
       $options = [];
       foreach ($views as $view_name => $view) {
@@ -66,6 +78,7 @@ class SearchBlock extends BlockBase {
         '#default_value' => $this->configuration['search_submit_label'],
         '#maxlength' => 255,
       ];
+    }
     
     return $form;
   }