Issue #2245783 by tim.plunkett: Regression: Help blocks display on 403/404 page.

core/modules/help/lib/Drupal/help/Tests/HelpTest.php

@@ -88,6 +88,15 @@ public function testHelp() {
    *   An HTTP response code.
    */
   protected function verifyHelp($response = 200) {
+    $this->drupalGet('admin/index');
+    $this->assertResponse($response);
+    if ($response == 200) {
+      $this->assertText('This page shows you all available administration tasks for each module.');
+    }
+    else {
+      $this->assertNoText('This page shows you all available administration tasks for each module.');
+    }
+
     foreach ($this->getModuleList() as $module => $name) {
       // View module help node.
       $this->drupalGet('admin/help/' . $module);

core/modules/system/lib/Drupal/system/Plugin/Block/SystemHelpBlock.php

@@ -91,8 +91,8 @@ public function access(AccountInterface $account) {
   protected function getActiveHelp(Request $request) {
     $output = '';
     $router_path = $request->attributes->get('_system_path');
-    // We will always have a path unless we are on a 403 or 404.
-    if (!$router_path) {
+    // Do not show on a 403 or 404 page.
+    if ($request->attributes->has('exception')) {
       return '';
     }