Question 267942 in Flutter Question posted in
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flutter : Exception has occurred. _TypeError (Null check operator used on a null value) I use appwrite library

if I use :

 uid: user!.uid,

and try to click the post button in my app this error happened :

 Exception has occurred. _TypeError (Null check operator used on a null value)

if I try what they say in debug console

: Error: Property 'uid' cannot be accessed on 'UserModel?' because it is potentially null.
lib/…/controller/post_controller.dart:106
- 'UserModel' is from 'package:studyv/models/user_model.dart' ('lib/models/user_model.dart').
package:studyv/models/user_model.dart:1
Try accessing using ?. instead.
      uid: user.uid,
                ^^^

and write it

 uid: user?.uid,

this error happened ?:

The argument type 'String?' can't be assigned to the parameter type 'String'.

I will cry!! what should I do when I try to solve it I end up making more errors in my app if someone knows what to do please help !!!!

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2

Answers


  1. 0

    I think you need to understand how Dart’s null safety works and for that I recommend you read the docs.

    The first error you had was because you were trying to read an ‘uid’ property out of a null UserModel Object.

    The second error happens because you’re trying to set ‘uid’ (which is of type ‘String’ – meaning it can’t be null) with UserModel’s ‘uid’ which is of type String? (can be null).

    String? canBeNull;
String cannotBeNull;

// This works because String? let's you take null values
canBeNull = null;
  
// This WILL FAIL because: A value of type 'Null' can't be assigned to a variable of type 'String'.
cannotBeNull = null; 

// This will work because if 'canBeNull' is not null, its value will be used, otherwise it will get 'defaultValue'
cannotBeNull = canBeNull ?? 'defaultValue';

    The best thing you can do is to check if ‘user.uid != null’ before making that attribution;

    Other option (not ideal) is to do something like this:

    uid: user?.uid ?? 'some fallback value'

    This will just make your compilation errors go away, but it won’t solve your problem because apparently your UserModel is getting a null value.

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  2. 0

    Looks like you don’t understand the null safety in Dart, you have to learn about it.

    When you use uid: user!.uid, it gives you the error because the user instance is null, maybe you didn’t initialize it well. And when you use uid: user?.uid, it allows you to affect a null to the property uid. You have two solutions, first is to make sure that you initialized the user instance before using it somewhere and use !. with it no problem, second use ?. with ?? like this : uid: user?.uid ?? 'default id here'. I don’t think you should use the second one because it is a user id. so you must use the first one and make sure that the user instance is initialized before you use it. Provide how you initialize it will help figuring a solution for your problem.
    to understand more about null safety this video may help.
    And don’t cry you just need to get things right that’s it.

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