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Flutter Null Context

I’m getting the following message when trying to display a message to the user

[ERROR:flutter/runtime/dart_vm_initializer.cc(41)] Unhandled Exception: Null check operator used on a null value
#0      Navigator.of (package:flutter/src/widgets/navigator.dart:2794)
#1      showDialog (package:flutter/src/material/dialog.dart:1425)
#2      _MyAppState.showMessage (package:simple_sensor_test/main.dart:397)
#3      _MyAppState.valueChange (package:simple_sensor_test/main.dart:101)
#4      _MyAppState.myField.<anonymous closure> (package:simple_sensor_test/main.dart:226)
#5      _FocusState._handleFocusChanged (package:flutter/src/widgets/focus_scope.dart:643)
#6      ChangeNotifier.notifyListeners (package:flutter/src/foundation/change_notifier.dart:437)
#7      FocusNode._notify (package:flutter/src/widgets/focus_manager.dart:1090)
#8      FocusManager.applyFocusChangesIfNeeded (package:flutter/src/widgets/focus_manager.dart:1871)
#9      _microtaskLoop (dart:async/schedule_microtask.dart:40)
#10     _startMicrotaskLoop (dart:async/schedule_microtask.dart:49)

The function which shows the message is

  Future<void> showMessage(String title, String message) async {
    if (mounted) {
      showDialog(
          context: context,
          builder: (context) =>
              Center(
                child: Material(
                  color: Colors.transparent,
                  child: Text('Message'),
                ),
              )
      );
    }
  }

I assume that the context is inherited from the State object: however when it gets to the Navigator function, it’s obviously not set. Any ideas, anyone?

Tags:

2

Answers


  1. 0

    Mounted is used to check if the widget is still part of the widget tree. However, mounted only tells you whether the widget is alive, but not if the context is valid.

    Ensure the context is valid at the time of use. Like this:

    Future<void> showMessage(String title, String message) async {
  // Ensure the widget is still mounted and context is valid
  if (mounted && context != null) {
    showDialog(
      context: context,
      builder: (BuildContext context) {
        return AlertDialog(
          title: Text(title),
          content: Text(message),
          actions: <Widget>[
            TextButton(
              onPressed: () {
                Navigator.of(context).pop();
              },
              child: Text('OK'),
            ),
          ],
        );
      },
    );
  }
}
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  2. 0

    Navigator.of(context) must be called at MaterialApp or Navigator tree. Check showMessage function is it in the right context.

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