0.87b4 MacOSX: Crash when copying folder in treeview

[iMiKE]

XnView: MP 0.87b4 - 64 bit
OS: macOS 10.12.6 64bit

XnView opened in Thumbnails+Filenames view. From the right of thumbs there is Treeview.
I clicked on empty folder in treeview, dragged it into folder with files (JPGs) - green plus appears meaning we're gonna copy folder, not move - released folder, and then XnView crashed.

Effect: Crash

To reproduce:

1. Open thumbnails view;
2. In treeview drag one empty folder into another (copy);
3. XnView crashed;

Actual behaviour (bug): XnView crashed

Expected behaviour: Copy one folder into another

Please, check attached logs

XnViewMP 0.87b4 crash.txt.zip

[xnview]

ok, i've found something strange, so please check with 0.87 final if the bug is fixed