message [nfc]: Add _disposed flag; check it

This change should have no user-facing effect.  The one spot where we
have an `if (_disposed)` check in editMessage prevents a state update
and a rebuild from happening.  This only applies if the store is
disposed before the edit request fails, but the MessageListView with
the edited message should get rebuilt anyway (through onNewStore)
when that happens.

Author: PIG208

lib/model/message.dart

@@ -94,12 +94,16 @@ class MessageStoreImpl extends PerAccountStoreBase with MessageStore {
 
   @override
   void registerMessageList(MessageListView view) {
+    assert(!_disposed);
     final added = _messageListViews.add(view);
     assert(added);
   }
 
   @override
   void unregisterMessageList(MessageListView view) {
+    // TODO: Add `assert(!_disposed);` here once we ensure [PerAccountStore] is
+    //   only disposed after [MessageListView]s with references to it are
+    //   disposed.  See [dispose] for details.
     final removed = _messageListViews.remove(view);
     assert(removed);
   }
@@ -122,6 +126,8 @@ class MessageStoreImpl extends PerAccountStoreBase with MessageStore {
     }
   }
 
+  bool _disposed = false;
+
   void dispose() {
     // Not disposing the [MessageListView]s here, because they are owned by
     // (i.e., they get [dispose]d by) the [_MessageListState], including in the
@@ -137,10 +143,14 @@ class MessageStoreImpl extends PerAccountStoreBase with MessageStore {
     //   [InheritedNotifier] to rebuild in the next frame) before the owner's
     //   `dispose` or `onNewStore` is called.  Discussion:
     //     https://chat.zulip.org/#narrow/channel/243-mobile-team/topic/MessageListView.20lifecycle/near/2086893
+
+    assert(!_disposed);
+    _disposed = true;
   }
 
   @override
   Future<void> sendMessage({required MessageDestination destination, required String content}) {
+    assert(!_disposed);
     // TODO implement outbox; see design at
     //   https://chat.zulip.org/#narrow/stream/243-mobile-team/topic/.23M3881.20Sending.20outbox.20messages.20is.20fraught.20with.20issues/near/1405739
     return _apiSendMessage(connection,
@@ -152,6 +162,7 @@ class MessageStoreImpl extends PerAccountStoreBase with MessageStore {
 
   @override
   void reconcileMessages(List<Message> messages) {
+    assert(!_disposed);
     // What to do when some of the just-fetched messages are already known?
     // This is common and normal: in particular it happens when one message list
     // overlaps another, e.g. a stream and a topic within it.
@@ -185,6 +196,7 @@ class MessageStoreImpl extends PerAccountStoreBase with MessageStore {
     required String originalRawContent,
     required String newContent,
   }) async {
+    assert(!_disposed);
     if (_editMessageRequests.containsKey(messageId)) {
       throw StateError('an edit request is already in progress');
     }
@@ -202,6 +214,8 @@ class MessageStoreImpl extends PerAccountStoreBase with MessageStore {
     } catch (e) {
       // TODO(log) if e is something unexpected
 
+      if (_disposed) return;
+
       final status = _editMessageRequests[messageId];
       if (status == null) {
         // The event actually arrived before this request failed
@@ -216,6 +230,7 @@ class MessageStoreImpl extends PerAccountStoreBase with MessageStore {
 
   @override
   ({String originalRawContent, String newContent}) takeFailedMessageEdit(int messageId) {
+    assert(!_disposed);
     final status = _editMessageRequests.remove(messageId);
     _notifyMessageListViewsForOneMessage(messageId);
     if (status == null) {