compose_box [nfc]: Reorder two calls in edit-message "Save" tap handler

Making sure to capture some data that store.editMessage needs from
the controller before that data is destroyed, in
composeBoxState.endEditInteraction, which disposes the controller.

Motivation: soon we'd like `store.editMessage` to return a Future,
which we'd like to await and add some error feedback, just like we
have in the send-message UX.

Author: chrisbobbe

lib/widgets/compose_box.dart

@@ -1815,11 +1815,13 @@ class _EditMessageBanner extends _Banner {
       return;
     }
 
+    final messageId = controller.messageId;
+    final newContent = controller.content.textNormalized;
+    composeBoxState.endEditInteraction();
     store.editMessage(
-      messageId: controller.messageId,
+      messageId: messageId,
       originalRawContent: originalRawContent,
-      newContent: controller.content.textNormalized);
-    composeBoxState.endEditInteraction();
+      newContent: newContent);
   }
 
   @override