Redirect to the correct URL.

Observer/DisableFrontend.php

@@ -75,8 +75,13 @@ public function execute(ObserverInterface $observer)
             $this->redirect->redirect($controller->getResponse(), $this->helperBackend->getHomePageUrl());
         }
         elseif ($configValue['show_frontend_as'] === 'redirect_to') {
-            // Check that the URL is valid.
-            // Redirect to that URL.
+            if (empty($configValue['redirect_to'])) {
+                // If there is no redirect destination, then throw an exception.
+                throw new \LogicException('No redirect destination was found.');
+            }
+
+            // The URL is valid. If it isn't, then the config form can't save.
+            $this->redirect->redirect($controller->getResponse(), $configValue['redirect_to']);
         }
     }
 }

etc/adminhtml/system.xml

@@ -10,8 +10,9 @@
                 </field>
                 <field id="redirect_to" translate="label" type="text" sortOrder="2" showInDefault="1" showInWebsite="0" showInStore="0">
                     <label>Redirect to</label>
+                    <validate>validate-url</validate>
                 </field>
             </group>
         </section>
     </system>
-</config>
+</config>

etc/module.xml

@@ -2,7 +2,7 @@
 <config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Module/etc/module.xsd">
     <module name="FocusriteNovation_DisableFrontend" setup_version="1.2.0">
         <sequence>
-            <module name="Magento_Store"/>
+            <module name="Magento_Store" />
         </sequence>
     </module>
-</config>
+</config>