StreamBuilder<FileSystemEntity>(dirty, state: _StreamBuilderBaseState<FileSystemEntity, AsyncSnapshot<FileSystemEntity>>#25d1b) - flutter

Question

I have got an error while working with Stream builders.
In my code, I used if(snapshot.data != null) { list.add(snapshot.data); } but I m getting a msg that add was called on null. I even added print('') statement to check if snapshot.data is null, but it is working fine with print() too. So I m not at all understanding how to solve this error. Someone please respond. Thanks in advance.
Here is my code
class Wrapper extends StatefulWidget {
#override
_WrapperState createState() => _WrapperState();
}
class _WrapperState extends State<Wrapper> {
Stream<FileSystemEntity> mystream;
var _directory;
void getfilesdata() async
{
_directory = await getApplicationDocumentsDirectory();
await Directory('${_directory.path}/data').create(recursive: true);
mystream = Directory('${_directory.path}/data').list().map((data) => data);
await Directory('${_directory.path}/documents').create(recursive: true);
}
#override
void initState() {
super.initState();
getfilesdata();
}
#override
Widget build(BuildContext context) {
List<FileSystemEntity> alldata;
return StreamBuilder<FileSystemEntity>(
stream: mystream,
builder: (context, snapshot) {
if(!snapshot.hasData)
{
print('No data yet');
return Loading();
}
else
{
if(snapshot.data != null && !snapshot.hasError)
{
print(snapshot.data);
alldata.add(snapshot.data);
}
if(snapshot.connectionState==ConnectionState.done)
{
return HomeScreen(alldata: alldata);
}
else return Loading();
}
}
);
}
}
And this is the result:
Performing hot restart...
Syncing files to device ZUK Z2132...
Restarted application in 1,734ms.
I/flutter (11840): No data yet
I/flutter (11840): No data yet
I/flutter (11840): File: '/data/user/0/com.example.easyscan/app_flutter/data/data[0]'
════════ Exception caught by widgets library ═══════════════════════════════════════════════════════
The following NoSuchMethodError was thrown building StreamBuilder<FileSystemEntity>(dirty, state: _StreamBuilderBaseState<FileSystemEntity, AsyncSnapshot<FileSystemEntity>>#25d1b):
The method 'add' was called on null.
Receiver: null
Tried calling: add(Instance of '_File')
The relevant error-causing widget was:
StreamBuilder<FileSystemEntity> file:///home/praneeth/AndroidStudioProjects/easyscan/lib/wrapper.dart:40:12
When the exception was thrown, this was the stack:
#0 Object.noSuchMethod (dart:core-patch/object_patch.dart:53:5)
#1 _WrapperState.build.<anonymous closure> (package:easyscan/wrapper.dart:53:25)
#2 StreamBuilder.build (package:flutter/src/widgets/async.dart:509:81)
#3 _StreamBuilderBaseState.build (package:flutter/src/widgets/async.dart:127:48)
#4 StatefulElement.build (package:flutter/src/widgets/framework.dart:4619:28)
...
════════════════════════════════════════════════════════════════════════════════════════════════════

Answer 1

Your error is because allData is null so you cannot call the add() method.
You can declare your variable as follow:
List<FileSystemEntity> alldata = [];
By doing this allData will be initialized as an empty list and you will be able to call its methods.