Description

When calling generateImages() on the Firebase Vertex AI Imagen model, we're seeing intermittent null pointer exceptions. The error occurs approximately 50% of the time when the function is called.

Error Details

flutter: Image generation error: NoSuchMethodError: The getter 'isEmpty' was called on null.
Receiver: null
Tried calling: isEmpty
#0      Object.noSuchMethod (dart:core-patch/object_patch.dart:38:5)
#1      new ImagenGenerationResponse.fromJson (package:firebase_vertexai/src/imagen_content.dart:105:21)
#2      parseImagenGenerationResponse (package:firebase_vertexai/src/imagen_content.dart:154:10)
#3      ImagenModel.generateImages.<anonymous closure> (package:firebase_vertexai/src/imagen_model.dart:95:13)

Steps to Reproduce

1. Initialize the Firebase Vertex AI Imagen model with the following configuration:

final model = FirebaseVertexAI.instance.imagenModel(
  model: 'imagen-3.0-fast-generate-001',
  generationConfig: ImagenGenerationConfig(
    numberOfImages: 1,
    aspectRatio: ImagenAspectRatio.landscape16x9,
    imageFormat: ImagenFormat.jpeg(compressionQuality: 90),
    addWatermark: false,
  ),
);

2. Call await model.generateImages(prompt) with a valid text prompt

Expected Behavior

The generateImages() method should consistently return valid image data without throwing exceptions.

Actual Behavior

In approximately 50% of the calls, the method throws a NoSuchMethodError because it's trying to call isEmpty on a null value during response parsing in ImagenGenerationResponse.fromJson.

Environment

• Flutter (3.29.2)
• firebase_vertexai package (1.4.0)

Possible Causes

Looking at the stack trace, the issue is in ImagenGenerationResponse.fromJson at line 105 in imagen_content.dart. The code is trying to access a property and call isEmpty on it, but is receiving null instead of the expected object or array.

Suggested Fix

The library should add null safety checks in the fromJson method to handle cases where the expected field is missing or null in the API response.