Additional form IDs are now correctly saved as arrays, not strings.

See #3115119

readonlymode.install

@@ -27,3 +27,33 @@ function readonlymode_update_8001() {
     'site_readonly_forms_viewonly' => 'forms.additional.view',
   ]);
 }
+
+/**
+ * Convert any existing additional form configuration from strings to arrays.
+ */
+function readonlymode_update_8002() {
+
+  /** @var \Drupal\Core\Config\Config */
+  $config = \Drupal::configFactory()->getEditable('readonlymode.settings');
+
+  foreach (['forms.additional.edit', 'forms.additional.view'] as $key) {
+
+    /** @var string|array */
+    $value = $config->get($key);
+
+    if (!\is_string($value)) {
+      continue;
+    }
+
+    $config->set($key, \preg_split('/\s+/m', $value));
+
+  }
+
+  $config->save();
+
+  // Delete the cached configuration in case updates were run via Drush with the
+  // '--no-cache-clear' flag so that the updated configuration gets used
+  // immediately.
+  \Drupal::cache('config')->delete('readonlymode.settings');
+
+}