mvc: try to fix crash report

(cherry picked from commit a14da189)
(cherry picked from commit 2b0cb7be)

src/opnsense/mvc/app/models/OPNsense/Base/FieldTypes/InterfaceField.php

@@ -129,12 +129,14 @@ class InterfaceField extends BaseField
             $allInterfacesDevices = array(); // mapping device -> interface handle (lan/wan/optX)
             $configObj = Config::getInstance()->object();
             // Iterate over all interfaces configuration and collect data
+            if (isset($configObj->interfaces) && $configObj->interfaces->count() > 0) {
                 foreach ($configObj->interfaces->children() as $key => $value) {
                     $allInterfaces[$key] = $value;
                     if (!empty($value->if)) {
                         $allInterfacesDevices[(string)$value->if] = $key;
                     }
                 }
+            }
 
             if ($this->internalAddParentDevices) {
                 // collect parents for lagg/vlan interfaces