getting "cannot handle the request because a modal dialog or alert is active," but can't find modal window

I'm using InDesign CC 2018 and am trying to open 7 files by dragging them to the InDesign icon. It opens 6 of them and then I get the "cannot handle the request because a modal dialog or alert is active" pop-up. The problem is that I can't find a modal window, so I can't close it.

I know that's vague but that's exactly what's happening. I'm on a 2017 iMac running High Sierra.

Thanks,

Lloyd