compose: Ensure topic gets set after message input focused.

To ensure the component is shown, wait a 'safe' time of 200ms.

Also, for consistency, change topic using `handleTopicChange`
rather than a direct `setState`.

src/compose/ComposeBox.js

@@ -131,12 +131,15 @@ class ComposeBox extends PureComponent<Props, State> {
   };
 
   handleMessageFocus = () => {
+    const { topic } = this.state;
     const { lastMessageTopic } = this.props;
-    this.setState(({ topic }) => ({
+    this.setState({
       isMessageFocused: true,
       isMenuExpanded: false,
-      topic: topic || lastMessageTopic,
-    }));
+    });
+    setTimeout(() => {
+      this.handleTopicChange(topic || lastMessageTopic);
+    }, 200); // wait, to hope the component is shown
   };
 
   handleMessageBlur = () => {