FIX: crash when loading stl file for modifier

Change-Id: If358e1a8bca908c2c4dda4a95510163867f56eb4

src/slic3r/GUI/GUI_ObjectList.cpp

@@ -1990,6 +1990,7 @@ void ObjectList::load_modifier(const wxArrayString& input_files, ModelObject& mo
             }
         }
 
+        model.add_default_instances();
         TriangleMesh mesh = model.mesh();
         // Mesh will be centered when loading.
         ModelVolume* new_volume = model_object.add_volume(std::move(mesh), type);