type 'List' is not a subtype of type 'List' 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: [
        // list of different widgets 
        .
        .
        .
      ].where((child) => child != null).toList(growable: true) as List,
    ),
  );
),

The error message says:

The following _CastError was thrown building LayoutBuilder:
type 'List' is not a subtype of type 'List' 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?

user_6618622 · Answer

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

List nullableWidgets = [];
List nonNullable = nullableWidgets.whereType().toList();

To answer your question:

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

user_179715 · Answer

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

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

user_11958566 · Answer

Not a Dart expert.

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

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

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

3 Answers