修复转发/切换话题时应用整体白屏崩溃

componentDidUpdate 的 initMessage 分支直接访问 this.messageEditArea
未判空，而输入框在 noInput（转发中）或 disabled（ACS未解析完）时会
被卸载——此时 initMessage（草稿/编辑内容）一变化就会解引用 null，
无 error boundary 兜底，整个应用白屏。

第十轮最小粒度测试 Ch3-4（实测复现2次）与 Ch8（同一处代码路径独立
命中）各自发现同一个bug。

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/widgets/send-message.jsx

@@ -228,9 +228,14 @@ class SendMessage extends React.PureComponent {
     } else if (prevProps.initMessage != this.props.initMessage) {
       const msg = this.props.initMessage || '';
       this.setState({message: msg}, _ => {
+        // messageEditArea can be null here: initMessage (draft/edit content) can change while the
+        // textarea itself is unmounted, e.g. noInput during an active forward, or disabled while
+        // ACS is still resolving right after a topic switch.
+        if (this.messageEditArea) {
           // If there is text, scroll to bottom and set caret to the end.
           this.messageEditArea.scrollTop = this.messageEditArea.scrollHeight;
           this.messageEditArea.setSelectionRange(msg.length, msg.length);
+        }
       });
     }
     if (prevProps.reply != this.props.reply) {