iphone app won't close, despite swiping attempts

I ended a phone conversation on my iPhone X (latest iOS), but the phone app window remains open and won't close despite every kind of swiping. The other open apps close properly, so it's something with this particular one. Worse, still, I can't reboot the phone by pressing the side buttons.

Apple suggests connecting the phone to my computer, but I fear there may be something nefarious going on and I don't want to expose my computer.