Fix #1704 (#1705)

So I think that the underlying issue is actually a system order ambiguity thing between `spawn_bonus` and `rotate_bonus`, but I'm not confident enough about `run_criteria`, `FixedTimeStep`, etc. to sort that out.

Author: rparrett

examples/game/alien_cake_addict.rs

@@ -330,11 +330,12 @@ fn spawn_bonus(
 // let the cake turn on itself
 fn rotate_bonus(game: Res<Game>, time: Res<Time>, mut transforms: Query<&mut Transform>) {
     if let Some(entity) = game.bonus.entity {
-        let mut cake_transform = transforms.get_mut(entity).unwrap();
-        cake_transform.rotate(Quat::from_rotation_y(time.delta_seconds()));
-        cake_transform.scale = Vec3::splat(
-            1.0 + (game.score as f32 / 10.0 * time.seconds_since_startup().sin() as f32).abs(),
-        );
+        if let Ok(mut cake_transform) = transforms.get_mut(entity) {
+            cake_transform.rotate(Quat::from_rotation_y(time.delta_seconds()));
+            cake_transform.scale = Vec3::splat(
+                1.0 + (game.score as f32 / 10.0 * time.seconds_since_startup().sin() as f32).abs(),
+            );
+        }
     }
 }