错误：主体可能正常完成，导致返回“null”，但返回类型“Widget”可能是不可为 null 的类型

Question

I'm having problems with my code inserting a "widget builder". An error appears in the line key builder: (BuildContext context, AsyncSnapshot<QuerySnapshot> snapshot) {

My code that is giving this failure is:

Widget build(BuildContext context) {
return StreamBuilder<QuerySnapshot>(
  stream: FirebaseFirestore.instance.collection("Conversas").snapshots(),
    builder: (BuildContext context, AsyncSnapshot<QuerySnapshot> snapshot) {
      if (snapshot.hasError) {
        return Center(
            child: Text("Desculpe. Aconteceu algum erro de nossa parte. =(")
        ); //Center
      }

      if(snapshot.connectionState == ConnectionState.waiting){
        return Center(
            child: Text("Carregando")
        ); //Center
      }

      if(snapshot.hasData) {
        return CustomScrollView(
          slivers: [
            CupertinoSliverNavigationBar(
              largeTitle: Text('Conversas'),
            ), //CupertinoSliverNavigationBar
            SliverList(
                delegate: SliverChildListDelegate(
                  snapshot.data!.docs.map((DocumentSnapshot document){
                return Container();
              }).toList())) //SliverChildListDelegate, SliverList
          ],
        ); //CustomScrollView
      }
}); //StreamBuilder

When I try to run the app, the log that appears in the console is:

Launching lib\main.dart on SM G780G in debug mode... Running Gradle task 'assembleDebug'... lib/screens/conversas.dart:13:18: Error: A non-null value must be returned since the return type 'Widget' doesn't allow null.

• 'Widget' is from 'package:flutter/src/widgets/framework.dart' ('/C:/src/flutter/packages/flutter/lib/src/widgets/framework.dart'). builder: (BuildContext context, AsyncSnapshot snapshot) { ^

FAILURE: Build failed with an exception.

• Where: Script 'C:\src\flutter\packages\flutter_tools\gradle\flutter.gradle' line: 1156

• What went wrong: Execution failed for task ':app:compileFlutterBuildDebug'.

Process 'command 'C:\src\flutter\bin\flutter.bat'' finished with non-zero exit value 1

• Try:

Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.

• Get more help at https://help.gradle.org

BUILD FAILED in 17s Exception: Gradle task assembleDebug failed with exit code 1

Answer 1

Return a container or sized box at the end. If no condition is met then it's null which is not a return type widget. But the method requires a widget to be returned so adding a return any widget at the end should resolve it

Widget build(BuildContext context) {
return StreamBuilder<QuerySnapshot>(
  stream: FirebaseFirestore.instance.collection("Conversas").snapshots(),
    builder: (BuildContext context, AsyncSnapshot<QuerySnapshot> snapshot) {
      if (snapshot.hasError) {
        return Center(
            child: Text("Desculpe. Aconteceu algum erro de nossa parte. =(")
        ); //Center
      }

      if(snapshot.connectionState == ConnectionState.waiting){
        return Center(
            child: Text("Carregando")
        ); //Center
      }

      if(snapshot.hasData) {
        return CustomScrollView(
          slivers: [
            CupertinoSliverNavigationBar(
              largeTitle: Text('Conversas'),
            ), //CupertinoSliverNavigationBar
            SliverList(
                delegate: SliverChildListDelegate(
                  snapshot.data!.docs.map((DocumentSnapshot document){
                return Container();
              }).toList())) //SliverChildListDelegate, SliverList
          ],
        ); //CustomScrollView
      }
return SizedBox.shrink();// add this. If it doesn't meet any condition then a sized box with no size is returned
}); //