Flutter null safety conditional widget - flutter

Question

Before Flutter introduced the null-safety feature, I was able to conditionally add Widgets within a list like so:
actions: <Widget>[
canCancel
? CupertinoDialogAction(
child: Text(cancelActionText),
onPressed: () {
Navigator.pop(context);
},
)
: null,
].where(notNull).toList()
notNull being a homemade filter that filters off the null objects...
Now with null-safety it's impossible because the list of Widgets strictly has to be non-null.
What would be a better approach?

Answer 1

Just use if inside the List:
<Widget>[
if (true) Widget(),
]
Example with your code:
actions: <Widget>[
if (canCancel)
CupertinoDialogAction(
child: Text(cancelActionText),
onPressed: () {
Navigator.pop(context);
},
),
]

Answer 2

Just replace your null with an empty, size zero, SizedBox.
SizedBox(width: 0, height: 0)
Or as suggested in comments:
SizedBox.shrink()

Answer 3

As YoBo suggested, using collection-if is the better approach here, but if for some reason you need to be able to store nulls in a List and filter them out later (or if you just prefer your existing style), you can:
Change your List type to allow nullable elements. That is, use <Widget?>[] instead of <Widget>[].
Use Iterable.whereType with a non-nullable type to filter out null values.
actions: <Widget?>[
canCancel
? CupertinoDialogAction(
child: Text(cancelActionText),
onPressed: () {
Navigator.pop(context);
},
)
: null,
].whereType<Widget>().toList();

Answer 4

if is not new as it was added in Dart 2.3 a few years ago. Even before null-safety, you were not allowed to return a null for a widget. You might not see a compile-time warning before NNBD, but it is a runtime error.
As I mentioned in this answer (although that answer isn't for your case), you can use if condition or even ternary operator like this:
Column(
children: [
if (condition) Text(''),
condition ? Text('') : Container(),
],
)