[BUGFIX] Fix access to non existent key in array (#1222)

Co-authored-by: Sebastian Meyer <[email protected]>

Classes/Controller/AbstractController.php

@@ -259,7 +259,7 @@ protected function sanitizeRequestData(): void
         }
 
         // tx_dlf[double] may only be 0 or 1.
-        $this->requestData['double'] = MathUtility::forceIntegerInRange($this->requestData['double'], 0, 1, 0);
+        $this->requestData['double'] = MathUtility::forceIntegerInRange($this->requestData['double'] ?? 0, 0, 1);
     }
 
     /**