Null check operator used on a null even if the ? is already added-CodePudding

I have 2 newbie questions here.

1st question: Why it won't go through the statement print('it will NOT go here.');

I got the error Null check operator used on a null value. But I already added ? to say that it can be null in the Price? class.

2nd question: Why this statement if (_price != null) shows The operand can't be null, so the condition is always true. But I know that it can be null since it is possible that it is not existing. Please help me understand what's going on. Thanks!

This is my code:

DocumentReference<Map<String, dynamic>> docRefPrice =
db.collection('dataPath').doc('ThisIsNotExisting'); 
print('it will go here - 1'); 
final DocumentSnapshot<Map<String, dynamic>> documentSnapshotPrice = await docRefPrice.get(); 
print('it will go here - 2'); 
Price? _price = Price.fromFirestore(documentSnapshotPrice); 
print('it will NOT go here.'); 
if (_price != null) { 
  print('_price is not null'); 
} 
else { print('_price is null'); 
  return false; 
}

Edit add fromFirestore:

  factory Price.fromFirestore(DocumentSnapshot<Map<String, dynamic>> doc) {
    Map data = doc.data()!;

    return Price(
      price1: data['text1'],
      price2: data['text2'],
    );
  }

This is the link that I tried to understand: https://dart.dev/codelabs/dart-cheatsheet

CodePudding user response：

Regarding your first question, the function fromFirestore is what's causing the error, add the code for it to your question.

the second question, _price won't be null cause Price.fromFirestore(documentSnapshotPrice); is a constructor so it will return a Price object and never null.

Edit

Most probably this line is the reason for the error: Map data = doc.data()!; remove the ! and make the Map nullable and handle this case

I will need the Price class code to write the code but here is the idea

factory Price.fromFirestore(DocumentSnapshot<Map<String, dynamic>> doc) {
Map? data = doc.data();

return Price(
  price1: data == null ? null : data['text1'], // price1 must accept null in this case
  price2: data == null ? null : data['text2'], // price2 must accept null in this case
);
}