'Bad state: No element' Error - Flutter Future Builder - flutter

Question

I am receiving an error that says:
Bad state: No element
The relevant error-causing widget was:
Expanded file:///C:/Users/***/AndroidStudioProjects/***/lib/ui/screens/price-screen.dart:142:23
The error is referring to this part of my code:
Expanded(
child: (snapshot.connectionState ==
ConnectionState.waiting)
? Container()
: Graph(snapshot.data, crypto, graphType))
I have snapshot.data equal to 'futureData', which is equal to pricesAndTimes. I wrote some print statements in getData() to see if pricesAndTimes is null and it is not null. I'm confused as to why I'm getting this error.
import 'package:bitcoin_ticker/data/coin-data.dart';
import 'package:bitcoin_ticker/ui/components/crypto-card.dart';
import 'package:bitcoin_ticker/ui/components/graph.dart';
import 'package:flutter/material.dart';
import 'package:flutter/cupertino.dart';
import '../../constants.dart';
import 'package:crypto_font_icons/crypto_font_icons.dart';
import '../components/toggle-button.dart';
class PriceScreen extends StatefulWidget {
#override
PriceScreenState createState() => PriceScreenState();
}
class PriceScreenState extends State<PriceScreen> {
String currency = 'USD';
String graphType = '1M';
String crypto = 'LTC';
Map<String, List<String>> pricesAndTimes = {};
Future<Map> futureData;
var isSelectedCrypto = <bool>[false, false, true];
var isSelectedGraph = <bool>[false, false, true, false, false];
var isSelectedCurrency = <bool>[true, false, false];
Future<Map> getData() async {
try {
pricesAndTimes = await CoinData(currency, graphType).getCoinData();
print("pricesAndTimes");
print(pricesAndTimes);
} catch (e) {
print(e);
}
return pricesAndTimes;
}
#override
void initState() {
super.initState();
futureData = getData();
}
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text('Crypto Watcher'),
),
body: FutureBuilder<Object>(
future: futureData,
builder: (context, snapshot) {
return Stack(
children: <Widget>[
Column(
mainAxisAlignment: MainAxisAlignment.spaceBetween,
crossAxisAlignment: CrossAxisAlignment.stretch,
children: <Widget>[
Center(
child: ToggleButtons(
borderWidth: 10,
children: <Widget>[
CryptoCard(
currency,
snapshot.connectionState == ConnectionState.waiting
? '---'
: pricesAndTimes['currentCoinPrices'][0],
'Bitcoin',
Icon(CryptoFontIcons.BTC),
),
CryptoCard(
currency,
snapshot.connectionState ==
ConnectionState.waiting
? '---'
: pricesAndTimes['currentCoinPrices'][1],
'Ethereum',
Icon(CryptoFontIcons.ETH)),
CryptoCard(
currency,
snapshot.connectionState ==
ConnectionState.waiting
? '---'
: pricesAndTimes['currentCoinPrices'][2],
'Litecoin',
Icon(CryptoFontIcons.LTC)),
],
onPressed: (int index) {
setState(() {
for (int buttonIndex = 0;
buttonIndex < isSelectedCrypto.length;
buttonIndex++) {
if (buttonIndex == index) {
isSelectedCrypto[buttonIndex] = true;
crypto = cryptoAbbreviation[buttonIndex];
} else {
isSelectedCrypto[buttonIndex] = false;
}
}
});
futureData = getData();
},
isSelected: isSelectedCrypto,
selectedColor: Colors.amber,
renderBorder: false,
fillColor: Colors.transparent,
)),
Center(
child: ToggleButtons(
borderWidth: 0.0,
splashColor: null,
renderBorder: false,
children: <Widget>[
ToggleButton('1D'),
ToggleButton('5D'),
ToggleButton('1M'),
ToggleButton('1Y'),
ToggleButton('5Y'),
],
onPressed: (int index) {
setState(() {
for (int buttonIndex = 0;
buttonIndex < isSelectedGraph.length;
buttonIndex++) {
if (buttonIndex == index) {
isSelectedGraph[buttonIndex] = true;
graphType = graphTypeList[buttonIndex];
} else {
isSelectedGraph[buttonIndex] = false;
}
}
});
futureData = getData();
},
isSelected: isSelectedGraph,
),
),
Expanded(
child: (snapshot.connectionState ==
ConnectionState.waiting)
? Container()
: Graph(snapshot.data, crypto, graphType)),
Center(
child: ToggleButtons(
borderWidth: 0.0,
splashColor: null,
renderBorder: false,
children: <Widget>[
ToggleButton('USD'),
ToggleButton('EUR'),
ToggleButton('GBP'),
],
onPressed: (int index) {
setState(() {
for (int buttonIndex = 0;
buttonIndex < isSelectedCurrency.length;
buttonIndex++) {
if (buttonIndex == index) {
isSelectedCurrency[buttonIndex] = true;
currency = currenciesList[buttonIndex];
} else {
isSelectedCurrency[buttonIndex] = false;
}
}
futureData = getData();
});
},
isSelected: isSelectedCurrency,
)),
]),
Center(
child: (snapshot.connectionState == ConnectionState.waiting)
? CircularProgressIndicator()
: Container())
],
);
}),
);
}
}

Answer 1

This error happens when your stream return a null value. To make sure the data sent to UI is not null, you can check if the snapshot has data. If not, return a loading indicator. Something like this:
FutureBuilder<Object>(
future: futureData,
builder: (context, snapshot) {
if (!snapshot.hasData) return Center(child: CircularProgressIndicator());
return Stack(
children: <Widget>[
// ... other lines
);

Answer 2

I realized that the issue doesn't lie in the code I posted above, but in the referenced Graph file from
Expanded(
child: (snapshot.connectionState ==
ConnectionState.waiting)
? Container()
: Graph(snapshot.data, crypto, graphType))
I forgot to check that file.