MacMail Preview Pane becomes blank

I am using Mac OS 10.13.2 on my late 2016 MacBook Pro. While I am reading an email in the preview pane of Mail, the contents of the email will suddenly disappear from the preview pane. It's as if no email from the list above has been selected. I can easily get it back by clicking on that email in the list; but it's annoying to have to do this. It usually happens to the most recent email & usually immediately after the Mail program is opened. It also happened on earlier versions of High Sierra. Any suggestions?