core/modules/system/src/Form/ThemeExperimentalConfirmForm.php | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/core/modules/system/src/Form/ThemeExperimentalConfirmForm.php b/core/modules/system/src/Form/ThemeExperimentalConfirmForm.php
index 67035dd07c..f838846544 100644
--- a/core/modules/system/src/Form/ThemeExperimentalConfirmForm.php
+++ b/core/modules/system/src/Form/ThemeExperimentalConfirmForm.php
@@ -107,8 +107,9 @@ public function buildForm(array $form, FormStateInterface $form_state) {
    * {@inheritdoc}
    */
   public function submitForm(array &$form, FormStateInterface $form_state) {
-    $theme = $form_state->getBuildInfo()['args'][0] ? $form_state->getBuildInfo()['args'][0] : NULL;
-    $set_default = $form_state->getBuildInfo()['args'][1] ? $form_state->getBuildInfo()['args'][1] : FALSE;
+    $args = $form_state->getBuildInfo()['args'];
+    $theme = isset($args[0]) ? $args[0] : NULL;
+    $set_default = isset($args[1]) ? $args[1] : FALSE;
     $themes = $this->themeHandler->rebuildThemeData();
     $config = $this->configFactory()->getEditable('system.theme');
     try {