type 'List<Widget?>' is not a subtype of type 'List<Widget>' in type cast

Question

After running dart migrate & applying null safety, that error popped up at my code and I think that's the error causing code block.

LayoutBuilder(builder: (context, cons) {
  return GestureDetector(
    child: new Stack(
      children: <Widget?>[
        // list of different widgets 
        .
        .
        .
      ].where((child) => child != null).toList(growable: true) as List<Widget>,
    ),
  );
),

The error message says:

The following _CastError was thrown building LayoutBuilder:
type 'List<Widget?>' is not a subtype of type 'List<Widget>' in type cast

The relevant error-causing widget was
LayoutBuilder
package:projectName/…/components/fileName.dart:182

If anyone encountered this issue, how it can be solved?

The children widgets of the stack goes red when I remove the " ? " Error message is: `the element type 'container?' can't be assigned to list type 'Widget'. for example @lrsvmb — Mahmoud Hassan Al-Ridini
– Mahmoud Hassan Al-Ridini, Commented Mar 5, 2021 at 19:38
Due to null safety restrictions removing all of them would cause another problem. — Mahmoud Hassan Al-Ridini
– Mahmoud Hassan Al-Ridini, Commented Mar 5, 2021 at 21:14

CopsOnRoad · Accepted Answer · 2021-05-22 14:35:34Z

4

Although there are different ways to solve it as provided here by @jamesdlin but the recommended one is to use whereType. For example:

List<Widget?> nullableWidgets = [];
List<Widget> nonNullable = nullableWidgets.whereType<Widget>().toList();

To answer your question:

Stack(
  children: <Widget?>[
    // Widgets (some of them nullable)
  ].whereType<Widget>().toList(),
)

answered May 22, 2021 at 14:35

CopsOnRoad

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jamesdlin · Accepted Answer · 2021-05-04 23:22:20Z

1

You cannot use as to cast List<T?> to List<T> because they are not directly related types; you would need to cast the elements instead, using List<Widget>.from() or Iterable.cast<Widget>().

See Dart convert List<String?> to List nnbd for how to remove null elements from a List<T?> and get a List<T> result (and thus avoiding the need to cast later).

edited May 4, 2021 at 23:22

answered Mar 5, 2021 at 20:05

jamesdlin

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CopsOnRoad · Accepted Answer · 2021-07-15 14:37:16Z

0

Not a Dart expert.

But it seems like the compiler does not respect your null safety checkup.

I suggest creating a new List<Widget> and populate it with each item from List<Widget?> which is not null.

edited Jul 15, 2021 at 14:37

CopsOnRoad

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answered Mar 5, 2021 at 19:40

Shlomi Katriel

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