Getx controller return null after using it for one time

Question

I am working on an application (something like a quiz app to do some calculations). For each screen I have a controller and one of these screens is the result screen. Everything works fine in the first attempt, however, in the second attempt, when I finish the quiz, The results screen returns null and says that "null check operator used on a null value.

this is the thrown exception


════════ Exception caught by widgets library ═══════════════════════════════════
The following _CastError was thrown building _BodyBuilder:
Null check operator used on a null value

The relevant error-causing widget was
Scaffold
lib/…/view/result_screen.dart:27
When the exception was thrown, this was the stack
#0      GetBuilderState.initState
package:get/…/simple/get_state.dart:134
#1      StatefulElement._firstBuild
package:flutter/…/widgets/framework.dart:4770
#2      ComponentElement.mount
package:flutter/…/widgets/framework.dart:4605
...     Normal element mounting (19 frames)
#21     Element.inflateWidget
package:flutter/…/widgets/framework.dart:3662
#22     MultiChildRenderObjectElement.inflateWidget
package:flutter/…/widgets/framework.dart:6285
#23     MultiChildRenderObjectElement.mount
package:flutter/…/widgets/framework.dart:6296
...     Normal element mounting (111 frames)
#134    Element.inflateWidget
package:flutter/…/widgets/framework.dart:3662
#135    MultiChildRenderObjectElement.inflateWidget
package:flutter/…/widgets/framework.dart:6285
#136    MultiChildRenderObjectElement.mount
package:flutter/…/widgets/framework.dart:6296
...     Normal element mounting (9 frames)
#145    Element.inflateWidget
package:flutter/…/widgets/framework.dart:3662
#146    MultiChildRenderObjectElement.inflateWidget
package:flutter/…/widgets/framework.dart:6285
#147    MultiChildRenderObjectElement.mount
package:flutter/…/widgets/framework.dart:6296
...     Normal element mounting (170 frames)
#317    Element.inflateWidget
package:flutter/…/widgets/framework.dart:3662
#318    MultiChildRenderObjectElement.inflateWidget
package:flutter/…/widgets/framework.dart:6285
#319    Element.updateChild
package:flutter/…/widgets/framework.dart:3414
#320    RenderObjectElement.updateChildren
package:flutter/…/widgets/framework.dart:5717
#321    MultiChildRenderObjectElement.update
package:flutter/…/widgets/framework.dart:6308
#322    Element.updateChild
package:flutter/…/widgets/framework.dart:3401
#323    ComponentElement.performRebuild
package:flutter/…/widgets/framework.dart:4657
#324    StatefulElement.performRebuild
package:flutter/…/widgets/framework.dart:4805
#325    Element.rebuild
package:flutter/…/widgets/framework.dart:4322
#326    StatefulElement.update
package:flutter/…/widgets/framework.dart:4837
#327    Element.updateChild
package:flutter/…/widgets/framework.dart:3401
#328    ComponentElement.performRebuild
package:flutter/…/widgets/framework.dart:4657
#329    Element.rebuild
package:flutter/…/widgets/framework.dart:4322
#330    ProxyElement.update
package:flutter/…/widgets/framework.dart:4985
#331    Element.updateChild
package:flutter/…/widgets/framework.dart:3401
#332    ComponentElement.performRebuild
package:flutter/…/widgets/framework.dart:4657
#333    Element.rebuild
package:flutter/…/widgets/framework.dart:4322
#334    ProxyElement.update
package:flutter/…/widgets/framework.dart:4985
#335    _InheritedNotifierElement.update
package:flutter/…/widgets/inherited_notifier.dart:181
#336    Element.updateChild
package:flutter/…/widgets/framework.dart:3401
#337    SingleChildRenderObjectElement.update
package:flutter/…/widgets/framework.dart:6154
#338    Element.updateChild
package:flutter/…/widgets/framework.dart:3401
#339    ComponentElement.performRebuild
package:flutter/…/widgets/framework.dart:4657
#340    StatefulElement.performRebuild
package:flutter/…/widgets/framework.dart:4805
#341    Element.rebuild
package:flutter/…/widgets/framework.dart:4322
#342    StatefulElement.update
package:flutter/…/widgets/framework.dart:4837
#343    Element.updateChild
package:flutter/…/widgets/framework.dart:3401
#344    SingleChildRenderObjectElement.update
package:flutter/…/widgets/framework.dart:6154
#345    Element.updateChild
package:flutter/…/widgets/framework.dart:3401
#346    SingleChildRenderObjectElement.update
package:flutter/…/widgets/framework.dart:6154
#347    Element.updateChild
package:flutter/…/widgets/framework.dart:3401
#348    ComponentElement.performRebuild
package:flutter/…/widgets/framework.dart:4657
#349    Element.rebuild
package:flutter/…/widgets/framework.dart:4322
#350    ProxyElement.update
package:flutter/…/widgets/framework.dart:4985
#351    Element.updateChild
package:flutter/…/widgets/framework.dart:3401
#352    ComponentElement.performRebuild
package:flutter/…/widgets/framework.dart:4657
#353    StatefulElement.performRebuild
package:flutter/…/widgets/framework.dart:4805
#354    Element.rebuild
package:flutter/…/widgets/framework.dart:4322
#355    BuildOwner.buildScope
package:flutter/…/widgets/framework.dart:2619
#356    WidgetsBinding.drawFrame
package:flutter/…/widgets/binding.dart:878
#357    RendererBinding._han
#358    SchedulerBinding._invokeFrameCallback
package:flutter/…/scheduler/binding.dart:1143
#359    SchedulerBinding.handleDrawFrame
package:flutter/…/scheduler/binding.dart:1080
#360    SchedulerBinding._handleDrawFrame
package:flutter/…/scheduler/binding.dart:996
#364    _invoke (dart:ui/hooks.dart:163:10)
#365    PlatformDispatcher._drawFrame (dart:ui/platform_dispatcher.dart:259:5)
#366    _drawFrame (dart:ui/hooks.dart:126:31)
(elided 3 frames from dart:async)

when I clicked on ( lib/…/view/result_screen.dart:27 ) in the error stack it took me to the scaffold of the screen. I am not sure why this is happening.

I have a HomeWrapper screen that consists of some widgets and a page view


void initControllers() {
    Get.put(HomeScreenController());
    Get.put(FirstPageController());
    Get.put(SecondPageController());
    Get.put(ThirdPageController());
    Get.put(FourthPageController());
    Get.put(FifthPageController());
    Get.put(SixthPageController());
    Get.put(SeventhPageController());
    Get.put(EighthPageController());
    Get.put(ResultsController());
  }

  @override
  Widget build(BuildContext context) {
    initControllers();
  return Column(...);

}

in the eighth page, there is a button that executes a method to caluculate the result and navigate to the results screen

class FinishButton extends StatelessWidget {
  final bool _isValid = Get.find<FirstPageController>().validLandArea;
  @override
  Widget build(BuildContext context) {
    return GetBuilder<ResultsController>(
      builder: (controller) => Padding(
        padding: const EdgeInsets.only(bottom: 20.0),
        child: CupertinoButton(
          child: Text('Finish'),
          onPressed: () {
            if (_isValid) {
              controller.clickFinishButton();
              Navigator.pushReplacementNamed(context, ResultScreen.id);
            } else {
              HomeScreenController.to.goToLandArea();
            }
          },
          color: AppColors.primaryColor,
        ),
      ),
    );
  }
}

in the results page, there is a button to take me back to the Root that will return either HomeWrapper or Authetication based on user value (whether he's logged in or not)

CupertinoButton(
                        child: Text('Back to Home'),
                        onPressed: () {
                          Navigator.pushReplacementNamed(context, Root.id);
                        },
                        color: AppColors.primaryColor,
                      ),

when I get back to HomeWrapper all pages are working fine with their controllers except when I navigate back to the result screen.

I tried to make the controller permenant and lazy put but did not work. If someone please can tell me what the problem is.

Try sharing your actual code and not just the error. In particular how you're initializing the controller and how you're navigating back to the page when you get the error. — Loren.A, Jun 29 '21 at 12:03
Are you using `GetMaterialApp` or `MaterialApp`? And are you seeing `[GETX]` logs of controllers being deleted when you navigate? You can try `pushNamed` instead of `pushReplacementNamed` when you route to the results screen. — Loren.A, Jun 29 '21 at 13:19
Yes I am using GetMaterialApp and I see all the getx logs. I just managed to solve the problem. Apparently, it was because of my PdfService controller (to save the result and share it) that i initialize using lazyPut in the main. I used "put" and it works just fine now but I have no Idea why. Can you please explain why this is happening if you got an idea? — Ali Alqallaf, Jun 29 '21 at 13:26
I'm pretty sure if you set the `fenix` property to `true` with lazyPut that also would have worked because it will return the same instance after being deleted. I suggest reading on the docs here https://github.com/jonataslaw/getx/blob/master/documentation/en_US/dependency_management.md#instancing-methods to have a better understanding of the different init methods. — Loren.A, Jun 29 '21 at 13:36

Getx controller return null after using it for one time

0 Answers0