Halcyon models don't support relations

Use a simple fill() approach instead of trying to set nested fields via their relations
Refs https://github.com/rainlab/translate-plugin/issues/216

modules/backend/traits/FormModelSaver.php

@@ -2,6 +2,8 @@
 
 use Str;
 use Backend\Classes\FormField;
+use October\Rain\Halcyon\Model as HalcyonModel;
+use October\Rain\Database\Model as DatabaseModel;
 
 /**
  * Form Model Saver Trait
@@ -50,6 +52,11 @@ trait FormModelSaver
             return;
         }
 
+        if ($model instanceof HalcyonModel) {
+            $model->fill($saveData);
+            return;
+        }
+
         $attributesToPurge = [];
         $singularTypes = ['belongsTo', 'hasOne', 'morphOne'];