Initialize modified masses resource

src/plugin/plugin.rs

@@ -181,6 +181,7 @@ where
         // Insert all of our required resources. Don’t overwrite
         // the `RapierConfiguration` if it already exists.
         app.init_resource::<RapierConfiguration>();
+        app.init_resource::<systems::ModifiedMasses>();
 
         app.insert_resource(SimulationToRenderTime::default())
             .insert_resource(RapierContext {

src/plugin/systems.rs

@@ -677,7 +677,8 @@ pub fn writeback_rigid_bodies(
 }
 
 /// Entities that likely had their mass properties changed this frame.
-#[derive(Resource, Deref, DerefMut)]
+#[derive(Resource, Deref, DerefMut, Default, Reflect)]
+#[reflect(Resource)]
 pub struct ModifiedMasses(pub Vec<Entity>);
 
 /// System responsible for writing updated mass properties back into the [`ReadMassProperties`] component.