Fix mesh extraction for meshes without associated material. (#18631)

# Objective

Fixes #17986
Fixes #18608

## Solution

Guard against situations where an extracted mesh does not have an
associated material. The way that mesh is dependent on the material api
(although decoupled) here is a bit unfortunate and we might consider
ways in the future to support these material features without this
indirect dependency.

Author: tychedelia

crates/bevy_pbr/src/meshlet/instance_manager.rs

@@ -1,10 +1,10 @@
 use super::{meshlet_mesh_manager::MeshletMeshManager, MeshletMesh, MeshletMesh3d};
 use crate::{
-    Material, MeshFlags, MeshTransforms, MeshUniform, NotShadowCaster, NotShadowReceiver,
-    PreviousGlobalTransform, RenderMaterialBindings, RenderMaterialInstances,
-    RenderMeshMaterialIds,
+    Material, MaterialBindingId, MeshFlags, MeshTransforms, MeshUniform, NotShadowCaster,
+    NotShadowReceiver, PreviousGlobalTransform, RenderMaterialBindings, RenderMaterialInstances,
+    RenderMeshMaterialIds, StandardMaterial,
 };
-use bevy_asset::{AssetEvent, AssetServer, Assets, UntypedAssetId};
+use bevy_asset::{AssetEvent, AssetId, AssetServer, Assets, UntypedAssetId};
 use bevy_ecs::{
     entity::{Entities, Entity, EntityHashMap},
     event::EventReader,
@@ -113,10 +113,16 @@ impl InstanceManager {
         };
 
         let mesh_material = mesh_material_ids.mesh_material(instance);
-        let mesh_material_binding_id = render_material_bindings
-            .get(&mesh_material)
-            .cloned()
-            .unwrap_or_default();
+        let mesh_material_binding_id =
+            if mesh_material != AssetId::<StandardMaterial>::invalid().untyped() {
+                render_material_bindings
+                    .get(&mesh_material)
+                    .cloned()
+                    .unwrap_or_default()
+            } else {
+                // Use a dummy binding ID if the mesh has no material
+                MaterialBindingId::default()
+            };
 
         let mesh_uniform = MeshUniform::new(
             &transforms,

crates/bevy_pbr/src/render/mesh.rs

@@ -1162,13 +1162,19 @@ impl RenderMeshInstanceGpuBuilder {
         // yet loaded. In that case, add the mesh to
         // `meshes_to_reextract_next_frame` and bail.
         let mesh_material = mesh_material_ids.mesh_material(entity);
-        let mesh_material_binding_id = match render_material_bindings.get(&mesh_material) {
-            Some(binding_id) => *binding_id,
-            None => {
-                meshes_to_reextract_next_frame.insert(entity);
-                return None;
-            }
-        };
+        let mesh_material_binding_id =
+            if mesh_material != AssetId::<StandardMaterial>::invalid().untyped() {
+                match render_material_bindings.get(&mesh_material) {
+                    Some(binding_id) => *binding_id,
+                    None => {
+                        meshes_to_reextract_next_frame.insert(entity);
+                        return None;
+                    }
+                }
+            } else {
+                // Use a dummy material binding ID.
+                MaterialBindingId::default()
+            };
         self.shared.material_bindings_index = mesh_material_binding_id;
 
         let lightmap_slot = match render_lightmaps.render_lightmaps.get(&entity) {