Mission Editor error after 2nd attempt to "Save WP File"

Using the Mission Editor on WSL1 I find that I can save the mission to a file only once. The second time I get the following error:

(mavproxy.py:3413): Gtk-WARNING **: Attempting to store changes into `/home/rmackay9/.local/share/recently-used.xbel', but failed: Failed to create file '/home/rmackay9/.local/share/recently-used.xbel.JCDAR0': No such file or directory

(mavproxy.py:3413): Gtk-WARNING **: Attempting to set the permissions of `/home/rmackay9/.local/share/recently-used.xbel', but failed: No such file or directory

(mavproxy.py:3413): Gtk-WARNING **: Attempting to store changes into `/home/rmackay9/.local/share/recently-used.xbel', but failed: Failed to create file '/home/rmackay9/.local/share/recently-used.xbel.S3W9Q0': No such file or directory

(mavproxy.py:3413): Gtk-WARNING **: Attempting to set the permissions of `/home/rmackay9/.local/share/recently-used.xbel', but failed: No such file or directory
Saved 8 waypoints to /home/rmackay9/spline-terr.wp
Traceback (most recent call last):
File "/usr/local/lib/python2.7/dist-packages/MAVProxy/modules/mavproxy_misseditor/missionEditorFrame.py", line 492, in save_wp_file_pushed
wx.FD_SAVE | wx.FD_OVERWRITE_PROMPT)
File "/usr/lib/python2.7/dist-packages/wx-3.0-gtk2/wx/_windows.py", line 3139, in __init__
_windows_.FileDialog_swiginit(self,_windows_.new_FileDialog(*args, **kwargs))
wx._core.PyAssertionError: C++ assertion "volDummy.empty() && pathDummy.empty()" failed at ../src/common/filename.cpp(568) in Assign(): the file name shouldn't contain the path

Reproduce the failure in the Mission Editor on WSL1 by saving a waypoint file twice. Start at MAVProxy/modules/mavproxy_misseditor/missionEditorFrame.py, especially save_wp_file_pushed around line 492, and inspect the wx.FileDialog arguments and traceback. Done means the second save completes without the reported wx assertion.

Written by the indexing model from the issue text.