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

I'm fairly certain this is NFC. The two calls do operate on shared
state, in the controller, but it should suffice to just capture
`messageId` and `newContent` from the ComposeBoxController before
endMessageEdit disposes it.

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

@@ -1789,11 +1789,13 @@ 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