Dart/Flutter problems with null-safety - firebase

Question

I got the same problem with null statements (?) in Dart multiple times in different cases. I really hope somebody can help me.
Just added some lines of code & the error:
Error:
The property 'isEmpty' can't be unconditionally accessed because the receiver can be 'null'. Try making the access conditional (using '?.') or adding a null check to the target ('!'). here
Here is one of my examples:
child: MaterialButton(
onPressed: () {
var currentState = this._formKey.currentState;
if (currentState == null) {
return;
}
if (_formKey.currentState.validate()) {
AuthService.instance.signIn(
email: emailTextEditingController.text,
password:
passwordTextEditingController.text);
if (AuthService.instance
.checkIfUserExists() ==
true) {
Navigator.pushReplacement(
context,
MaterialPageRoute(
builder: (context) => MainMenu()));
} else {
Navigator.pushReplacement(
context,
MaterialPageRoute(
builder: (context) =>
VerifyScreen()));
}
}
},
Got this error-message again:
The method 'validate' can't be unconditionally invoked because the receiver can be 'null'.Try making the call conditional (using '?.') or adding a null check to the target ('!').
After I edited the code with a ! to avoid the Null-Statement like:
singUpUser() {
if (formKey.currentState!.validate()) {
setState(() {
isLoading = true;
});
} else {
return null;
};
But now i just avoid the error in the code itself, after starting a emulator and testing it, next error appears:
Null check operator used on a null value
So thats not the right solution...
If you need more code, just message me.
Thank you!
Tom

Answer 1

In a nutshell: if Dart is certain that a variable at compile time can be null at runtime, it doesn't compile.. unless you explicitly check for null values, and/or promote the variable to be non-nullable with the ! operator (Dart is not always able to infer the non-nullability in certain situations, so it's our responsibility to promote them to non-nullable).
There's much more to know if you're curious ("why?", for starters), so I'd suggest to check the null safety documentation (quick read).
This being said, your code now changes:
(1) We must check if val is nullable, before accessing it. We can either use ! or .? to safely access it; note: the null check operator ! is the least safe null operator, it's likely that it will result in run time exceptions.
validator: (val) {
val==null || val?.isEmpty || val?.length<3
? "Enter Username 3+ characters"
: null
}
(2) I can't infer which method / variable can be null by myself
(3) It depends on what you're trying to do, but I guess that you're trying to implement the Firebase authentication process, in which your user can and should be null before authenticating. Therefore, your function should accept a nullable user value (User?). In there, we do the usual null check and we add a ! operator to promote its value in case user is not null. As aforementioned, Dart isn't always able to infer nullability of variables.
MyUser _userFromFirebaseUser(User? user) {
return user==null ? null : MyUser(userId: user!.uid);
}
Note how using a null-check ! here is perfectly safe, only because you just checked its nullability in the same line (nonetheless, keep a wise-eye when you refactor this).
EDIT. (4)
I can't infer where exactly your exception is fired, but since you want to validate your form, then here's my code from a project of mine:
// inside "saveForm()"...
var currentState = this._formKey.currentState;
if (currentState == null)
return; // this just means something went wrong
if (!currentState.validate()) return; // todo if invalid, handle this, maybe show a snackbar and stuff...
Note how the variable currentState is now promoted to be non-nullable WITHOUT using the null check operator !, which is just good practice (avoid using ! whenever possible, PREFER using null-aware operators, such as ?. or ??, or ?=)

Answer 2

Being empty is not the same as being null. So before you can check an object is empty, you need to check against null first.
if (obj != null && !obj.isEmpty) {}