Don't panic when StandardMaterial normal_map hasn't loaded yet (bevye…

…ngine#5307)

# Objective

[This unwrap()](https://github.com/bevyengine/bevy/blob/de484c1e4147d01bf34c88a10797b75128a0d98a/crates/bevy_pbr/src/pbr_material.rs#L195) in pbr_material.rs will be hit if a StandardMaterial normal_map image has not finished loading, resulting in an error message that is hard to debug.

## Solution

~~This PR improves the error message including a potential indication of why the unwrap() could have panic'd by using expect() instead of unwrap().~~

This PR removes the panic by only proceeding if the image is found.

---

## Changelog

Don't panic when StandardMaterial normal_map images have not finished loading.

crates/bevy_pbr/src/pbr_material.rs

@@ -190,19 +190,17 @@ impl AsBindGroupShaderType<StandardMaterialUniform> for StandardMaterial {
  }
  let has_normal_map = self.normal_map_texture.is_some();
  if has_normal_map {
- match images
- .get(self.normal_map_texture.as_ref().unwrap())
- .unwrap()
- .texture_format
- {
- // All 2-component unorm formats
- TextureFormat::Rg8Unorm
- | TextureFormat::Rg16Unorm
- | TextureFormat::Bc5RgUnorm
- | TextureFormat::EacRg11Unorm => {
- flags |= StandardMaterialFlags::TWO_COMPONENT_NORMAL_MAP;
+ if let Some(texture) = images.get(self.normal_map_texture.as_ref().unwrap()) {
+ match texture.texture_format {
+ // All 2-component unorm formats
+ TextureFormat::Rg8Unorm
+ | TextureFormat::Rg16Unorm
+ | TextureFormat::Bc5RgUnorm
+ | TextureFormat::EacRg11Unorm => {
+ flags |= StandardMaterialFlags::TWO_COMPONENT_NORMAL_MAP;
+ }
+ _ => {}
  }
- _ => {}
  }
  if self.flip_normal_map_y {
  flags |= StandardMaterialFlags::FLIP_NORMAL_MAP_Y;