Flutter 错误列的子项不得包含任何 null 值，但在索引 0 处找到 null 值

Question

I was trying a camera app in flutter and encountered the issue that column's children must not contain null values, but a null value was found at index 0. I am not able to pass the widget in column's children.

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

void main() {
  runApp(new MaterialApp(
    title: "Camera App",
    home: LandingScreen(),
    ));
}

class LandingScreen extends StatefulWidget {
  @override
  _LandingScreenState createState() => _LandingScreenState();
}

class _LandingScreenState extends State<LandingScreen> {

  File imageFile;

  _openGallery(BuildContext context) async {
    var picture = await ImagePicker.pickImage(source: ImageSource.gallery);
    this.setState((){
      imageFile = picture;
    });
    Navigator.of(context).pop();
  }

  _openCamera(BuildContext context) async{
    var picture = await ImagePicker.pickImage(source: ImageSource.camera);
    this.setState((){
      imageFile = picture;
    });
    Navigator.of(context).pop();
  }

  Future<void> _showChoiceDialog(BuildContext context){
    return showDialog(context: context,builder: (BuildContext context){
      return AlertDialog(
        title: Text("Make a choice!"),
        content: SingleChildScrollView(
          child: ListBody(
            children: <Widget>[
              GestureDetector(
                child: Text("Gallery"),
                onTap: (){
                  _openGallery(context);
                },
                ),
                Padding(padding: EdgeInsets.all(8.0),),
                GestureDetector(
                child: Text("Camera"),
                onTap: (){
                  _openCamera(context);
                },
                ),
            ],
            ),
        ),
        );
    });
  }

  Widget _decideImageView(){
    if(imageFile == null){
      return Text("No Image Selected");
    } 
    else{
      return Image.file(imageFile, width: 400, height: 400,);
    }
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("Main Screen"),
      ),
      body: Container(
        child: Center(
          child: Column(
            mainAxisAlignment: MainAxisAlignment.spaceAround,
            children: <Widget>[
              _decideImageView(),
              RaisedButton(onPressed: (){
                _showChoiceDialog(context);
              },child: Text("Select Image!"),)
            ],
          )
          )
      )
    );
  }
}

I have checked with and without return in the else section but the issue is still there.

I have included the whole code now.

Answer 1

you have to return your Image Widget in your else case:

Widget _decideImageView(){
        if(imageFile == null){
          return Text("No Image Selected");
        } 
        else{
          return Image.file(imageFile, width: 400, height: 400,);
        }
      }

Answer 2

Just add a return before Image.File at else section of _decideImageView() function.

You have to keep in mind that the function _decideImageView() you have called must return something. So when your imageFile is not null it is going to the else but not returning. That is why you are getting the error of null index in column