compose: Refactor some logic; implement redesigned error banner

lib/widgets/action_sheet.dart

@@ -14,6 +14,7 @@ import '../model/narrow.dart';
 import 'actions.dart';
 import 'clipboard.dart';
 import 'color.dart';
+import 'compose_box.dart';
 import 'dialog.dart';
 import 'icons.dart';
 import 'inset_shadow.dart';
@@ -325,22 +326,22 @@ class QuoteAndReplyButton extends MessageActionSheetMenuItemButton {
   @override void onPressed() async {
     final zulipLocalizations = ZulipLocalizations.of(pageContext);
 
-    // This will be null only if the compose box disappeared after the
-    // message action sheet opened, and before "Quote and reply" was pressed.
-    // Currently a compose box can't ever disappear, so this is impossible.
-    var composeBoxController = findMessageListPage().composeBoxController!;
-    final topicController = composeBoxController.topicController;
+    var composeBoxController = findMessageListPage().composeBoxController;
+    // The compose box doesn't null out its controller; it's either always null
+    // (e.g. in Combined Feed) or always non-null; it can't have been nulled out
+    // after the action sheet opened.
+    composeBoxController!;
     if (
-      topicController != null
-      && topicController.textNormalized == kNoTopicTopic
+      composeBoxController is StreamComposeBoxController
+      && composeBoxController.topic.textNormalized == kNoTopicTopic
       && message is StreamMessage
     ) {
-      topicController.value = TextEditingValue(text: message.topic);
+      composeBoxController.topic.value = TextEditingValue(text: message.topic);
     }
 
     // This inserts a "[Quoting…]" placeholder into the content input,
     // giving the user a form of progress feedback.
-    final tag = composeBoxController.contentController
+    final tag = composeBoxController.content
       .registerQuoteAndReplyStart(PerAccountStoreWidget.of(pageContext),
         message: message,
       );
@@ -353,11 +354,11 @@ class QuoteAndReplyButton extends MessageActionSheetMenuItemButton {
 
     if (!pageContext.mounted) return;
 
-    // This will be null only if the compose box disappeared during the
-    // quotation request. Currently a compose box can't ever disappear,
-    // so this is impossible.
-    composeBoxController = findMessageListPage().composeBoxController!;
-    composeBoxController.contentController
+    composeBoxController = findMessageListPage().composeBoxController;
+    // The compose box doesn't null out its controller; it's either always null
+    // (e.g. in Combined Feed) or always non-null; it can't have been nulled out
+    // during the raw-content request.
+    composeBoxController!.content
       .registerQuoteAndReplyEnd(PerAccountStoreWidget.of(pageContext), tag,
         message: message,
         rawContent: rawContent,