fix null exception, add retained layer

com.unity.uiwidgets/Runtime/painting/text_style.cs

                 background: background ?? (backgroundColor != null
                     ? backgroundPaint : null
                 ),
-                shadows: shadows.Cast<Shadow>().ToList()
+                shadows: shadows?.Cast<Shadow>().ToList()
                 // fontFeatures: fontFeatures,
             );
         }

engine/src/lib/ui/compositing/scene_builder.cc

 }
 
 UIWIDGETS_API(void)
+SceneBuilder_addRetained(SceneBuilder* ptr, EngineLayer* retainedLayer) {
+  ptr->addRetained(static_cast<fml::RefPtr<EngineLayer>>(retainedLayer));
+}
+
+UIWIDGETS_API(void)
 SceneBuilder_pop(SceneBuilder* ptr) { ptr->pop(); }
 
 UIWIDGETS_API(Scene*) SceneBuilder_build(SceneBuilder* ptr) {