compose [nfc]: Make just one call to _ComposeBoxContainer

Author: chrisbobbe

lib/widgets/compose_box.dart

@@ -1291,31 +1291,32 @@ class _ComposeBoxState extends State<ComposeBox> implements ComposeBoxState {
 
   @override
   Widget build(BuildContext context) {
+    final Widget body;
+
     final errorBanner = _errorBanner(context);
     if (errorBanner != null) {
-      return _ComposeBoxContainer(child: errorBanner);
-    }
-
-    final narrow = widget.narrow;
-    final Widget body;
-    switch (narrow) {
-      case ChannelNarrow():
-        assert(_controller is StreamComposeBoxController);
-        body = _StreamComposeBoxBody(controller: (_controller as StreamComposeBoxController),
-          narrow: narrow);
-      case TopicNarrow():
-        assert(_controller is FixedDestinationComposeBoxController);
-        body = _FixedDestinationComposeBoxBody(controller: (_controller as FixedDestinationComposeBoxController),
-          narrow: narrow);
-      case DmNarrow():
-        assert(_controller is FixedDestinationComposeBoxController);
-        body = _FixedDestinationComposeBoxBody(controller: (_controller as FixedDestinationComposeBoxController),
-          narrow: narrow);
-      case CombinedFeedNarrow():
-      case MentionsNarrow():
-      case StarredMessagesNarrow():
-        assert(false);
-        body = const SizedBox.shrink();
+      body = errorBanner;
+    } else {
+      final narrow = widget.narrow;
+      switch (narrow) {
+        case ChannelNarrow():
+          assert(_controller is StreamComposeBoxController);
+          body = _StreamComposeBoxBody(controller: (_controller as StreamComposeBoxController),
+            narrow: narrow);
+        case TopicNarrow():
+          assert(_controller is FixedDestinationComposeBoxController);
+          body = _FixedDestinationComposeBoxBody(controller: (_controller as FixedDestinationComposeBoxController),
+            narrow: narrow);
+        case DmNarrow():
+          assert(_controller is FixedDestinationComposeBoxController);
+          body = _FixedDestinationComposeBoxBody(controller: (_controller as FixedDestinationComposeBoxController),
+            narrow: narrow);
+        case CombinedFeedNarrow():
+        case MentionsNarrow():
+        case StarredMessagesNarrow():
+          assert(false);
+          body = const SizedBox.shrink();
+      }
     }
 
     return _ComposeBoxContainer(child: body);