Fix panic in multiple_windows example (#1737)

Fixes #1736

There seemed to be an obvious transcription error where `setup_pipeline` was being called during `on_update` of `AppState::CreateWindow` instead of `AppState::Setup`.

However, the example still panicked after fixing that.

I'm not sure if there's some intended or unintended change in event processing during state transitions or something, but this PR make the example work again.

Cleaned up an unused `Stage` label while I was in there.

Please feel free to close this in favor of another approach.

Author: rparrett

examples/window/multiple_windows.rs

@@ -21,21 +21,17 @@ fn main() {
         .add_system_set(
             SystemSet::on_update(AppState::CreateWindow).with_system(setup_window.system()),
         )
-        .add_system_set(
-            SystemSet::on_enter(AppState::CreateWindow).with_system(setup_pipeline.system()),
-        )
+        .add_system_set(SystemSet::on_update(AppState::Setup).with_system(setup_pipeline.system()))
         .run();
 }
 
-#[derive(Debug, Hash, PartialEq, Eq, Clone, StageLabel)]
-pub struct Stage;
-
 // NOTE: this "state based" approach to multiple windows is a short term workaround.
 // Future Bevy releases shouldn't require such a strict order of operations.
 #[derive(Clone, Eq, PartialEq)]
 enum AppState {
     CreateWindow,
     Setup,
+    Done,
 }
 
 fn setup_window(
@@ -66,13 +62,18 @@ fn setup_pipeline(
     mut render_graph: ResMut<RenderGraph>,
     asset_server: Res<AssetServer>,
     msaa: Res<Msaa>,
+    mut app_state: ResMut<State<AppState>>,
 ) {
     // get the non-default window id
     let window_id = windows
         .iter()
         .find(|w| w.id() != WindowId::default())
-        .map(|w| w.id())
-        .unwrap();
+        .map(|w| w.id());
+
+    let window_id = match window_id {
+        Some(window_id) => window_id,
+        None => return,
+    };
 
     // here we setup our render graph to draw our second camera to the new window's swap chain
 
@@ -205,4 +206,6 @@ fn setup_pipeline(
         transform: Transform::from_xyz(6.0, 0.0, 0.0).looking_at(Vec3::ZERO, Vec3::Y),
         ..Default::default()
     });
+
+    app_state.set_next(AppState::Done).unwrap();
 }