Advert for XMind.AI pops up in XMind Pro

This morning, much to my horror, an advert for Xmind.AI Back to School special popped up while I was trying get some urgent work done.  This is not the free version, it's fully licenses PRO!!  Worse still, there is no close button.  Not even closing XMind from the taskbar closed the popup.  I had to end XMind from the task manager to get it to close!

What is going on??  Is XMind pestering it's PRO users with ads that they cannot close, or is there some kind of malfunction/bug that caused this?