Fix screenshot example (bevyengine#15094)

# Objective

I noticed some issues in `screenshot` example:
1. Cursor icon won't return from `SystemCursorIcon::Progress` to default
icon, even though screen shot saving is done.
2. Panics when exiting window: ``called `Result::unwrap()` on an `Err`
value:
NoEntities("bevy_ecs::query::state::QueryState<bevy_ecs::entity::Entity,
bevy_ecs::query::filter::With<bevy_window::window::Window>>")``

## Solution

1. Caused by cursor updating system not responding to [`CursorIcon`
component
removal](https://github.com/bevyengine/bevy/blob/5cfcbf47ed386515adefe80f99f8e8f7e7f3ce5a/examples/window/screenshot.rs#L38).
I believe it should, so change it to react to
`RemovedComponents<CursorIcon>`. (a suggestion)
2. Use `get_single` for window.

## Testing

- run screenshot example

---------

Co-authored-by: Alice Cecile <[email protected]>

crates/bevy_render/src/view/window/cursor.rs

@@ -4,10 +4,11 @@ use bevy_ecs::{
     change_detection::DetectChanges,
     component::Component,
     entity::Entity,
+    observer::Trigger,
     query::With,
     reflect::ReflectComponent,
     system::{Commands, Local, Query, Res},
-    world::Ref,
+    world::{OnRemove, Ref},
 };
 use bevy_reflect::{std_traits::ReflectDefault, Reflect};
 use bevy_utils::{tracing::warn, HashSet};
@@ -27,6 +28,8 @@ impl Plugin for CursorPlugin {
         app.register_type::<CursorIcon>()
             .init_resource::<CustomCursorCache>()
             .add_systems(Last, update_cursors);
+
+        app.observe(on_remove_cursor_icon);
     }
 }
 
@@ -84,12 +87,12 @@ pub enum CustomCursor {
 
 pub fn update_cursors(
     mut commands: Commands,
-    mut windows: Query<(Entity, Ref<CursorIcon>), With<Window>>,
+    windows: Query<(Entity, Ref<CursorIcon>), With<Window>>,
     cursor_cache: Res<CustomCursorCache>,
     images: Res<Assets<Image>>,
     mut queue: Local<HashSet<Entity>>,
 ) {
-    for (entity, cursor) in windows.iter_mut() {
+    for (entity, cursor) in windows.iter() {
         if !(queue.remove(&entity) || cursor.is_changed()) {
             continue;
         }
@@ -161,6 +164,15 @@ pub fn update_cursors(
     }
 }
 
+/// Resets the cursor to the default icon when `CursorIcon` is removed.
+pub fn on_remove_cursor_icon(trigger: Trigger<OnRemove, CursorIcon>, mut commands: Commands) {
+    commands
+        .entity(trigger.entity())
+        .insert(PendingCursor(Some(CursorSource::System(
+            convert_system_cursor_icon(SystemCursorIcon::Default),
+        ))));
+}
+
 /// Returns the image data as a `Vec<u8>`.
 /// Only supports rgba8 and rgba32float formats.
 fn image_to_rgba_pixels(image: &Image) -> Option<Vec<u8>> {

examples/window/screenshot.rs

@@ -32,7 +32,9 @@ fn screenshot_saving(
     screenshot_saving: Query<Entity, With<Capturing>>,
     windows: Query<Entity, With<Window>>,
 ) {
-    let window = windows.single();
+    let Ok(window) = windows.get_single() else {
+        return;
+    };
     match screenshot_saving.iter().count() {
         0 => {
             commands.entity(window).remove::<CursorIcon>();