Environment: Mac OS 10.13.6
Acrobat Pro DC version 2019.010.20064
1. Open 2 PDFs and tile them vertically
2. Activate the Comment app in both documents.
3. Collapse the Comment sidebars on both documents
4. Select some text and click on the Highlight tool in the commenting toolbar
5. Observe that both Comment sidebars expand automatically (undesirable behavior).
6. Type Command+W to close PDFs without saving.
Documents both close without a crash.
Rick Soldin commented
I am also having this problem but it a little more simplified to get to the crash.
Mac OS 10.14.3
Acrobat Pro DC v. 2019.010.20091
1. Open 2 PDFs
2. Activate Comment sidebar in 1 window
3. Type Cmd+W to close 1 window.