Exception thrown by widgets library '!children.any((Widget child) => child == null)': is not true

2
votes

Hi I am getting the following error while running the application What can be the possible reason for the issue?

══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
The following assertion was thrown building Home(dirty, state: _HomeState#f7a67):
'package:flutter/src/widgets/framework.dart': Failed assertion: line 1639: '!children.any((Widget
child) => child == null)': is not true.
3
widgetdartstack-traceflutter
What code causes this exception? Sounds like you're passing a list of widgets somewhere where one or more of the items in the list are null – Günter Zöchbauer
More detailes required to resolve the problem. Can you share some code if possible, especially the List causing the issue. Thank You – Hemanth Raj

3 Answers

10
votes

Faced the same problem a few days ago.

This happens when you forget to or miss to return the Widget.

Without seeing your code it's difficult to know the exact point of failure.

As far I am concerned, my previous code goes like:

@override
Widget build(BuildContext context) {
//....

  new Column(
  //.....
  );
}

And after fixing it::

@override
Widget build(BuildContext context) {
//....

  return new Column(
  //.....
  );
}

Hope this helps!

2
votes

Might also get this problem if there is null in list of widgets:

Widget build() {
  return new Column(
    children: [
      new Title(),
      new Body(),
      shouldShowFooter ? null : new Container()
    ]
  );
}

To solve this:

bool notNull(Object o) => o != null;
Widget build() {
  return new Column(
    children: <Widget>[
      new Title(),
      new Body(),
      shouldShowFooter ? new Footer() : new Container()
    ].where(notNull).toList(),
  );
}

More info on it: https://github.com/flutter/flutter/issues/3783

0
votes

In my case, the result of buildChildrenis null, or one element in the returned list is null.

Column column = new Column(
  // center the children
  mainAxisAlignment: MainAxisAlignment.start,
  crossAxisAlignment: CrossAxisAlignment.stretch,
  children: buildChildren(context),
);