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

@@ -129,10 +129,12 @@ class InterfaceField extends BaseField
             $allInterfacesDevices = array(); // mapping device -> interface handle (lan/wan/optX)
             $configObj = Config::getInstance()->object();
             // Iterate over all interfaces configuration and collect data
-            foreach ($configObj->interfaces->children() as $key => $value) {
-                $allInterfaces[$key] = $value;
-                if (!empty($value->if)) {
-                    $allInterfacesDevices[(string)$value->if] = $key;
+            if (isset($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;
+                    }
                 }
             }