Fix usage of wrong variable and add validation for n+1 port

src/Radio/Configuration.php

@@ -335,11 +335,11 @@ class Configuration
                     $backend_config = (array)$row['backend_config'];
 
                     if (!empty($backend_config['dj_port'])) {
-                        $port = (int)$frontend_config['dj_port'];
+                        $port = (int)$backend_config['dj_port'];
                         $used_ports[$port] = $station_reference;
                     }
                     if (!empty($backend_config['telnet_port'])) {
-                        $port = (int)$frontend_config['telnet_port'];
+                        $port = (int)$backend_config['telnet_port'];
                         $used_ports[$port] = $station_reference;
                     }
                 }

src/Validator/Constraints/StationPortCheckerValidator.php

@@ -53,6 +53,12 @@ class StationPortCheckerValidator extends ConstraintValidator
                     ->setParameter('{{ port }}', $port)
                     ->addViolation();
             }
+
+            if ($port_path === 'backend_config_dj_port' && isset($used_ports[$port+1])) {
+                $this->context->buildViolation($constraint->message)
+                    ->setParameter('{{ port }}', sprintf('%s (%s + 1)', $port+1, $port))
+                    ->addViolation();
+            }
         }
     }
 }