title: Error: The non-abstract class 'InternalSelectableMathState'
body: <p>I just updated flutter version from 2.5.3 to 2.8. I have the following error that i dont know how resolve it. There is no error on any plugin installed, It seems that the error comes from the inner classes themselves and I don't know in which part of my application the error is throwed:</p>
  <pre><code>../../AppData/Local/Pub/Cache/hosted/pub.dartlang.org/flutter_math_fork-0.3.3+1/lib/src/widgets/selectable.dart:407:7: Error: The non-abstract class 'InternalSelectableMathState' is missing implementations for these members:
   - TextSelectionDelegate.copySelection
   - TextSelectionDelegate.cutSelection
   - TextSelectionDelegate.pasteText
   - TextSelectionDelegate.selectAll
  Try to either
   - provide an implementation,
   - inherit an implementation from a superclass or mixin,
   - mark the class as abstract, or
   - provide a 'noSuchMethod' implementation.
  
  class InternalSelectableMathState extends State&lt;InternalSelectableMath&gt;
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  /C:/desarrollo/flutter/packages/flutter/lib/src/services/text_input.dart:985:8: Context: 'TextSelectionDelegate.copySelection' is defined here.
    void copySelection(SelectionChangedCause cause);
         ^^^^^^^^^^^^^
  /C:/desarrollo/flutter/packages/flutter/lib/src/services/text_input.dart:965:8: Context: 'TextSelectionDelegate.cutSelection' is defined here.
    void cutSelection(SelectionChangedCause cause);
         ^^^^^^^^^^^^
  /C:/desarrollo/flutter/packages/flutter/lib/src/services/text_input.dart:973:16: Context: 'TextSelectionDelegate.pasteText' is defined here.
    Future&lt;void&gt; pasteText(SelectionChangedCause cause);
                 ^^^^^^^^^
  /C:/desarrollo/flutter/packages/flutter/lib/src/services/text_input.dart:979:8: Context: 'TextSelectionDelegate.selectAll' is defined here.
    void selectAll(SelectionChangedCause cause);
         ^^^^^^^^^
  
  
  FAILURE: Build failed with an exception.
  
  * Where:
  Script 'C:\desarrollo\flutter\packages\flutter_tools\gradle\flutter.gradle' line: 1070
  
  * What went wrong:
  Execution failed for task ':app:compileFlutterBuildDebug'.
  &gt; Process 'command 'C:\desarrollo\flutter\bin\flutter.bat'' finished with non-zero exit value 1
  
  * Try:
  Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.
  
  * Get more help at https://help.gradle.org
  
  BUILD FAILED in 8m 5s
  Exception: Gradle task assembleDebug failed with exit code 1
  </code></pre>
  <p>I dont have declared <code>flutter_math_fork</code> on my pubspec.</p>
  <p>My flutter doctor output:</p>
  <blockquote>
  <pre><code>Doctor summary (to see all details, run flutter doctor -v):
  [√] Flutter (Channel stable, 2.8.0, on Microsoft Windows [VersiÃ³n 10.0.19041.1348], locale es-ES)
  [√] Android toolchain - develop for Android devices (Android SDK version 31.0.0)
  [√] Chrome - develop for the web
  [√] Android Studio (version 2020.3)
  [√] VS Code (version 1.62.3)
  [√] Connected device (3 available)
  
  • No issues found!
  </code></pre>
  </blockquote>
  <p>I have tried: flutter clean, flutter upgrade and invalidate cache / restart.</p>
  <p>Any suggestion is appreciated.</p>