Fix crash with disabled winit (#3330)

# Objective

This PR fixes a crash when winit is enabled when there is a camera in the world. Part of #3155

## Solution

In this PR, I removed two unwraps and added an example for regression testing.

Cargo.toml

@@ -228,6 +228,10 @@ path = "examples/app/return_after_run.rs"
 name = "thread_pool_resources"
 path = "examples/app/thread_pool_resources.rs"
 
+[[example]]
+name = "without_winit"
+path = "examples/app/without_winit.rs"
+
 # Assets
 [[example]]
 name = "asset_loading"

crates/bevy_core_pipeline/src/main_pass_3d.rs

@@ -47,10 +47,11 @@ impl Node for MainPass3dNode {
         world: &World,
     ) -> Result<(), NodeRunError> {
         let view_entity = graph.get_input_entity(Self::IN_VIEW)?;
-        let (opaque_phase, alpha_mask_phase, transparent_phase, target, depth) = self
-            .query
-            .get_manual(world, view_entity)
-            .expect("view entity should exist");
+        let (opaque_phase, alpha_mask_phase, transparent_phase, target, depth) =
+            match self.query.get_manual(world, view_entity) {
+                Ok(query) => query,
+                Err(_) => return Ok(()), // No window
+            };
 
         {
             // Run the opaque pass, sorted front-to-back

crates/bevy_pbr/src/light.rs

@@ -341,7 +341,10 @@ pub fn add_clusters(
     cameras: Query<(Entity, &Camera), Without<Clusters>>,
 ) {
     for (entity, camera) in cameras.iter() {
-        let window = windows.get(camera.window).unwrap();
+        let window = match windows.get(camera.window) {
+            Some(window) => window,
+            None => continue,
+        };
         let clusters = Clusters::from_screen_size_and_z_slices(
             UVec2::new(window.physical_width(), window.physical_height()),
             Z_SLICES,

examples/README.md

@@ -122,6 +122,7 @@ Example | File | Description
 `plugin_group` | [`app/plugin_group.rs`](./app/plugin_group.rs) | Demonstrates the creation and registration of a custom plugin group
 `return_after_run` | [`app/return_after_run.rs`](./app/return_after_run.rs) | Show how to return to main after the Bevy app has exited
 `thread_pool_resources` | [`app/thread_pool_resources.rs`](./app/thread_pool_resources.rs) | Creates and customizes the internal thread pool
+`without_winit` | [`app/without_winit.rs`](./app/without_winit.rs) | Create an application without winit (runs single time, no event loop)
 
 ## Assets
 

examples/app/without_winit.rs

@@ -0,0 +1,13 @@
+use bevy::prelude::*;
+use bevy::winit::WinitPlugin;
+
+fn main() {
+    App::new()
+        .add_plugins_with(DefaultPlugins, |group| group.disable::<WinitPlugin>())
+        .add_system(setup_system)
+        .run();
+}
+
+fn setup_system(mut commands: Commands) {
+    commands.spawn_bundle(PerspectiveCameraBundle::default());
+}