Fix get null of dirtTexture (#2235)

* fix: get null of dirtTexture

Author: zhuxudong

packages/loader/src/SceneLoader.ts

@@ -138,12 +138,14 @@ class SceneLoader extends Loader<Scene> {
               bloomEffect.dirtIntensity = postProcessData.bloom.dirtIntensity;
               tonemappingEffect.enabled = postProcessData.tonemapping.enabled;
               tonemappingEffect.mode = postProcessData.tonemapping.mode;
-              // @ts-ignore
-              // prettier-ignore
-              const dirtTexturePromise = resourceManager.getResourceByRef<any>( postProcessData.bloom.dirtTexture).then((texture) => {
-                bloomEffect.dirtTexture = texture;
-              });
-              promises.push(dirtTexturePromise);
+              if (postProcessData.bloom.dirtTexture) {
+                // @ts-ignore
+                // prettier-ignore
+                const dirtTexturePromise = resourceManager.getResourceByRef<any>(postProcessData.bloom.dirtTexture).then((texture) => {
+                    bloomEffect.dirtTexture = texture;
+                });
+                promises.push(dirtTexturePromise);
+              }
             }
 
             return Promise.all(promises).then(() => {