Description of problem:
I was comparing two very large folders (Windows user directories) stored on two different NTFS partitions, one stored in the local hard drive and one on an external drive.

Version-Release number of selected component:
meld-3.19.1-1.fc29

Additional info:
reporter:       libreport-2.9.7
cmdline:        /usr/bin/python3 /usr/bin/meld Users/gsvelto/AppData/ ../Windows/Users/gsvelto/AppData/
crash_function: isdir
exception_type: TypeError
executable:     /usr/bin/meld
interpreter:    python3-3.7.1-4.fc29.x86_64
kernel:         4.19.10-300.fc29.x86_64
runlevel:       N 5
type:           Python3
uid:            1000

Truncated backtrace:
genericpath.py:42:isdir:TypeError: stat: path should be string, bytes, os.PathLike or integer, not NoneType

Traceback (most recent call last):
  File "/usr/lib/python3.7/site-packages/meld/dirdiff.py", line 1085, in on_treeview_selection_changed
    it, pane, os_path)
  File "/usr/lib/python3.7/site-packages/meld/tree.py", line 124, in is_folder
    return icon == "folder" or os.path.isdir(path)
  File "/usr/lib64/python3.7/genericpath.py", line 42, in isdir
    st = os.stat(s)
TypeError: stat: path should be string, bytes, os.PathLike or integer, not NoneType

Local variables in innermost frame:
s: None