NoSuchMethodError: the method 'toRawHandle' was called on null.

[JCKodel]

AndroidManifest.xml:

<manifest xmlns:android="http://schemas.android.com/apk/res/android" package="br.art.code.xxx" 
    xmlns:tools="http://schemas.android.com/tools">
    <application tools:replace="android:label,android:icon,android:name" android:name=".App" android:label="xxx"
        android:icon="@mipmap/ic_launcher" android:roundIcon="@mipmap/icon_round" android:fullBackupContent="@xml/backup_rules">
        <receiver android:name="com.dexterous.flutterlocalnotifications.ScheduledNotificationBootReceiver">
            <intent-filter>
                <action android:name="android.intent.action.BOOT_COMPLETED"></action>
            </intent-filter>
        </receiver>

        <receiver android:name="com.dexterous.flutterlocalnotifications.ScheduledNotificationReceiver" />

        <meta-data android:name="com.facebook.sdk.ApplicationId" android:value="@string/facebook_app_id"/>
        <meta-data android:name="com.google.android.gms.ads.APPLICATION_ID" android:value="ca-app-pub-xxx~xxx"/>

        <activity android:name="com.yalantis.ucrop.UCropActivity" android:screenOrientation="portrait" 
            android:theme="@style/Theme.AppCompat.Light.NoActionBar"/>

        <activity android:name="com.facebook.FacebookActivity" android:configChanges="keyboard|keyboardHidden|screenLayout|screenSize|orientation"
            android:label="@string/app_name" />

        <activity android:name="com.facebook.CustomTabActivity" android:exported="true">
            <intent-filter>
                <action android:name="android.intent.action.VIEW" />
                <category android:name="android.intent.category.DEFAULT" />
                <category android:name="android.intent.category.BROWSABLE" />
                <data android:scheme="@string/fb_login_protocol_scheme" />
            </intent-filter>
        </activity>

        <activity android:name=".MainActivity" android:launchMode="singleTop" android:theme="@style/LaunchTheme" 
            android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|layoutDirection|fontScale|screenLayout|density|uiMode"
            android:hardwareAccelerated="true" android:windowSoftInputMode="adjustResize">
            <meta-data android:name="io.flutter.app.android.SplashScreenUntilFirstFrame" android:value="true" />
            <intent-filter>
                <action android:name="android.intent.action.MAIN"/>
                <category android:name="android.intent.category.LAUNCHER"/>
            </intent-filter>
        </activity>
    </application>

    <uses-permission android:name="android.permission.RECEIVE_BOOT_COMPLETED"/>
</manifest>

App.kt as documentation suggests.

[√] Flutter (Channel master, v1.10.7-pre.37, on Microsoft Windows [Version 10.0.18362.356], locale en-GB)
    • Flutter version 1.10.7-pre.37 at C:\flutter
    • Framework revision 56d68a9071 (30 hours ago), 2019-09-26 14:01:57 -0700
    • Engine revision b126ba7585
    • Dart version 2.6.0 (build 2.6.0-dev.3.0 6ff8d2199d)

 
[√] Android toolchain - develop for Android devices (Android SDK version 29.0.2)
    • Android SDK at C:\Users\jckod\Android\SDK
    • Android NDK location not configured (optional; useful for native profiling support)
    • Platform android-29, build-tools 29.0.2
    • ANDROID_HOME = C:\Users\jckod\Android\SDK
    • Java binary at: C:\Program Files\Android\Android Studio\jre\bin\java
    • Java version OpenJDK Runtime Environment (build 1.8.0_202-release-1483-b03)
    • All Android licenses accepted.

[√] Chrome - develop for the web
    • Chrome at C:\Program Files (x86)\Google\Chrome\Application\chrome.exe

[√] Android Studio (version 3.5)
    • Android Studio at C:\Program Files\Android\Android Studio
    • Flutter plugin version 39.0.3
    • Dart plugin version 191.8423
    • Java version OpenJDK Runtime Environment (build 1.8.0_202-release-1483-b03)

[√] VS Code (version 1.38.1)
    • VS Code at C:\Users\jckod\AppData\Local\Programs\Microsoft VS Code
    • Flutter extension version 3.4.1

[√] Connected device (3 available)
    • Android SDK built for x86 • emulator-5554 • android-x86    • Android 7.0 (API 24) (emulator)
    • Chrome                    • chrome        • web-javascript • Google Chrome 77.0.3865.90
    • Server                    • web           • web-javascript • Flutter Tools

• No issues found!

[JCKodel]

Note: This happens only on VSCode.
Android Studio starts the app (but my task is never triggered).

Continuing after the first error:

Exception has occurred.
PlatformException (PlatformException(1, You have not properly initialized the Flutter WorkManager Package. You should ensure you have called the 'initialize' function first! Example: 

`Workmanager.initialize(
  callbackDispatcher,
 )`

The `callbackDispatcher` is a top level function. See example in repository., null))

BTW, I've inserted this on my App.kt file: Log.d("WorkmanagerPlugin", "WorkmanagerPlugin is initializing") and this log appears on catlog when the app is initialized (before Flutter first frame is rendered and the bug manifests itself).

After running a couple of times (and moving this initialization first thing in my flutter main), the first error doesn't occur anymore, but the second remains:

I also have to write this: WidgetsFlutterBinding.ensureInitialized();, otherwise, my app says:

Exception has occurred.
FlutterError (ServicesBinding.defaultBinaryMessenger was accessed before the binding was initialized.
If you're running an application and need to access the binary messenger before `runApp()` has been called (for example, during plugin initialization), then you need to explicitly call the `WidgetsFlutterBinding.ensureInitialized()` first.
If you're running a test, you can call the `TestWidgetsFlutterBinding.ensureInitialized()` as the first line in your test's `main()` method to initialize the binding.)

[timrijckaert]

Is your callbackDispatcher a top level function?
Can you provide me with a sample that demonstrates this behaviour?

[JCKodel]

That was the problem. Moving the function to public and outside the class made it work.