The argument type 'Null' can't be assigned to the parameter type 'User'-CodePudding

I am new to firebase and am trying to use firebase authenticator. Currently I have 2 dart files - main.dart and user.dart.

In user.dart below, I am facing this issue with null safety.

On the last line, I am facing the error: A value of type 'Null' can't be assigned to a parameter of type 'User' in a const constructor.

So to resolve this, I decided to change line 7 to final User? data; which resolved the issue. However, the problem now arises in my main.dart file.

Here is a snapshot of my main.dart

On line 25, I receive the error: The argument type 'User?' can't be assigned to the parameter type 'User'.

So to fix this, I decided to change line 25 to (user) => CurrentUser.create(user!), . Adding the '!'

When I run my code now, I receive this exception:

══╡ EXCEPTION CAUGHT BY PROVIDER ╞══════════════════════════════════════════════════════════════════
The following assertion was thrown:
An exception was throw by _MapStream<User?, CurrentUser> listened by
StreamProvider<CurrentUser>, but no `catchError` was provided.

Exception:
Null check operator used on a null value

════════════════════════════════════════════════════════════════════════════════════════════════════

Can someone please explain what is going on and how I can solve this. Thanks!

CodePudding user response：

Once you set the field data in the class CurrentUser as nullable, of type User?, you also have to change the line 12 of your class, by setting the parameter received by the factory method create() as User?.

Like this:

factory CurrentUser.create(User? data){
  return CurrentUser._(data, false);
}

CodePudding user response：

You're getting these compile-time errors since you're trying to explicitly set a NON-NULLABLE variable to null, directly in a constructor.

Another thing that's worth mentioning is the use of the ! operator in your case. When you use the ! operator you're telling Dart that you, as the developer, are taking responsibility in making sure that the variable is not null. Therefore, since the variable you're passing is indeed null, a run-time error is thrown.

All of your implementation can be drastically simplified once you understand null safety. For example the isInitialValue parameter can be easily get rid of.

Take a good read at Dart's Sound Null Safety documentation to understand so.

CodePudding user response：

make your variable is nullable or add ! to your user variable to check null