[SuperEditor] Detach plugins in dispose(). (Resolves #2790)

super_editor/lib/src/default_editor/super_editor.dart

@@ -534,6 +534,10 @@ class SuperEditorState extends State<SuperEditor> {
       }
     }
 
+    for (final plugin in widget.plugins) {
+      plugin.detach(widget.editor);
+    }
+
     _iosControlsController.dispose();
     _androidControlsController.dispose();
 

super_editor/test/super_editor/supereditor_plugin_test.dart

@@ -0,0 +1,40 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:flutter_test_runners/flutter_test_runners.dart';
+import 'package:super_editor/super_editor.dart';
+
+import 'supereditor_test_tools.dart';
+
+void main() {
+  group('SuperEditor > plugins >', () {
+    testWidgetsOnAllPlatforms('are detached when the editor is disposed', (tester) async {
+      final plugin = _FakePlugin();
+
+      await tester //
+          .createDocument()
+          .withSingleParagraph()
+          .withPlugin(plugin)
+          .pump();
+
+      // Ensure the plugin was not attached initially.
+      expect(plugin.wasDetached, isFalse);
+
+      // Pump another widget tree to dispose SuperEditor.
+      await tester.pumpWidget(Container());
+
+      // Ensure the plugin was detached.
+      expect(plugin.wasDetached, isTrue);
+    });
+  });
+}
+
+/// A plugin that tracks whether it was detached.
+class _FakePlugin extends SuperEditorPlugin {
+  bool get wasDetached => _wasDetached;
+  bool _wasDetached = false;
+
+  @override
+  void detach(Editor editor) {
+    _wasDetached = true;
+  }
+}