fix(ui): don't save a draft into a disposed channel

packages/stream_chat_flutter/CHANGELOG.md

@@ -15,6 +15,7 @@
 
 🐞 Fixed
 
+- Fixed `StreamMessageComposer` throwing when its channel is disposed while it still holds unsent text, for example when the user is disconnected with a channel page open.
 - Fixed a message that failed to send, update or delete hiding the timestamp and delivery status of the message before it.
 - Fixed a voice recording sent as a reply without text showing inside a filled container instead of directly on the message bubble.
 - Fixed a mention rendering as another mentioned user's shorter mention followed by leftover text, such as `@Anna` next to `@Ann`.

packages/stream_chat_flutter/lib/src/message_input/stream_message_composer.dart

@@ -1730,6 +1730,9 @@ class DefaultStreamMessageComposerState extends State<DefaultStreamMessageCompos
     final channel = StreamChannel.maybeOf(context)?.channel;
     if (channel == null) return;
 
+    // A channel that isn't initialized yet or was disposed can't hold a draft.
+    if (channel.state == null) return;
+
     final message = _effectiveController.message;
     final isMessageValid = _effectiveController.validator.call(message);
 

packages/stream_chat_flutter/test/src/message_input/message_composer_draft_test.dart

@@ -16,7 +16,7 @@ void main() {
 
   // Stands up a composer with the given controller and a draft stream the
   // test can drive on demand.
-  Future<void> pumpComposer(
+  Future<MockChannel> pumpComposer(
     WidgetTester tester, {
     required StreamMessageComposerController composerController,
     required Stream<Draft?> draftStream,
@@ -72,6 +72,8 @@ void main() {
     // Let the post-frame callback fire and set up the draft stream listener
     // before the test starts emitting events.
     await tester.pumpAndSettle();
+
+    return channel;
   }
 
   group('draft stream', () {
@@ -198,4 +200,41 @@ void main() {
       expect(composerController.text, isEmpty);
     });
   });
+
+  group('when the composer is removed with unsent text', () {
+    testWidgets('saves the text as a draft', (tester) async {
+      final composerController = StreamMessageComposerController()..text = 'unsent';
+      addTearDown(composerController.dispose);
+
+      final channel = await pumpComposer(
+        tester,
+        composerController: composerController,
+        draftStream: const Stream.empty(),
+      );
+
+      await tester.pumpWidget(const SizedBox());
+
+      verify(() => channel.createDraft(any())).called(1);
+    });
+
+    testWidgets('does not throw once the channel is disposed', (tester) async {
+      final composerController = StreamMessageComposerController()..text = 'unsent';
+      addTearDown(composerController.dispose);
+
+      final channel = await pumpComposer(
+        tester,
+        composerController: composerController,
+        draftStream: const Stream.empty(),
+      );
+
+      // A disposed channel has no state, and throws on any call that needs it.
+      when(() => channel.state).thenReturn(null);
+      when(() => channel.createDraft(any())).thenThrow(StateError('disposed'));
+
+      await tester.pumpWidget(const SizedBox());
+
+      expect(tester.takeException(), isNull);
+      verifyNever(() => channel.createDraft(any()));
+    });
+  });
 }