Flutter image_pickerで最初に遭遇するエラー Failed assertion: line 853 pos 14: 'file != null': is not true.

画像が必ずセットされていないとエラーになる

初めてimage_pickerを使ってみると、大体発生するエラー。。。

こういうコードの場合、画像が何も選択されていないからエラーが発生します

════════ Exception caught by widgets library ═══════════════════════════════════
'package:flutter/src/painting/image_provider.dart': Failed assertion: line 853 pos 14: 'file != null': is not true.
The relevant error-causing widget was
UserImagePicker
lib/…/auth/auth_form.dart:60
════════════════════════════════════════════════════════════════════════════════

import 'dart:io';
import 'package:flutter/material.dart';
import 'package:image_picker/image_picker.dart';

class UserImagePicker extends StatefulWidget {
  @override
  _UserImagePickerState createState() => _UserImagePickerState();
}

class _UserImagePickerState extends State<UserImagePicker> {
  File _pickedImage;
  void _pickImage() async {
    final pickedImage =
        await ImagePicker().getImage(source: ImageSource.gallery);
    final pickedImageFile = File(pickedImage.path);
    setState(() {
      _pickedImage = pickedImageFile;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Column(
      children: <Widget>[
        CircleAvatar(
          radius: 40,
          backgroundImage: FileImage(_pickedImage),
        ),
        TextButton.icon(
          onPressed: _pickImage,
          icon: Icon(Icons.image),
          label: Text(
            'Add Image',
            style: TextStyle(color: Theme.of(context).primaryColor),
          ),
        ),
      ],
    );
  }
}

「nullの場合」を追加するだけで解決！

もしnullじゃなければ左（FileImage(_pickedImage)）、nullなら右（null ）

と言うような処理で解決です！

 CircleAvatar(
          radius: 40,
          backgroundImage:
              _pickedImage != null ? FileImage(_pickedImage) : null,
        ),