Warning implementing list filtering in Flutter

user_2665223 0
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I have the following code to filter a list from data to be shown in a listView:

child: FutureBuilder(
               future: fetchPacientes('todo'),
               builder: (context, snapshot) {
                 if (snapshot.hasData) {
                   var filteredList = snapshot.data;
                   print("a minusculas:" +
                       _controller.text.toLowerCase());
                   filteredList = filteredList
                       .where((element) => (element.nombre
                       .toLowerCase()
                       .contains(
                       _controller.text.toLowerCase()) ||
                       element.NHC.toLowerCase().contains(
                           _controller.text.toLowerCase()) ||
                       element.apellidos.toLowerCase().contains(
                           _controller.text.toLowerCase())))
                       .toList();

But I am getting an error at point:

.where(...

This is the error output:

The method 'where' can't be unconditionally invoked because the receiver can be 'null'.

I am trying to migrate an old app to Null Safety, but I am not able to solve this issue. The proposed solution is to add a null check at filteredList! but the error is not removed when doing that way

dart flutter
2 Answers

That error means that the object on which the . is applied (in this case, the filteredList) can be null, so it would throw an exception if filteredList will be actually null.

So, to avoid the error, you have these options:

  • Add a ! after filteredList: in this case, you're assuring the compiler that filteredList will never ever be null, and the error will disappear. Note that this doesn't actually prevent it from being null: you're just reassuring the compiler about it, and filteredList could still be null. In that case, the app will throw a dedicated exception telling you that it found a 'null' object when you declared in the code that this should not have happened
  • Check if filteredList is null: add a few lines of code testing if it is really null, and then handle the case, even something basic like returning a Text('filteredList is empty'). After this, you can safely write filteredList! because you are sure that it will never be null, since you actually tested id.
  • Set a default value for filteredList in case it's null: Dart has this operator that assigns a value to an object if it is null: filteredList ??= []. Again, after using this, you can safely write filteredList! because it will never be null.

PS sorry, didn't notice the last sentence. Since adding ! and a null check isn't working, I'd try setting a default value for filteredList. Or maybe checking for null on snapshot.data, and then set an explicit non-nullable type for filteredList, like List<Object> filteredList.

user_25018580

The error is becausesnapshot.data can be null. Try adding a ? before your .where like so :

filteredList = filteredList?.where(...);
user_124134040
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