Problem with MTPaint graphic editor on Lubuntu 18.4

[Sergio Belloro]

I posted a survey on https://forum.ubuntu-it.org/viewtopic.php?f=8&t=630991 about what was described below and an expert replied that in Lubuntu 18.4 there is a bug of Lubuntu Bionic 18.04 LTS which can appear in different applications, for example here https://bugs.launchpad.net/ubuntu/+sour ... ug / 1770129.
So I represent the problem I detected with the MTPaint 3.40 application in Lubuntu 18.4.1, hoping for your decisive intervention.
I state that, to open an image file, I usually use the double click on the file icon with the left mouse button (using the default Evince program).
If I want to modify an image file instead, I receive an error when I open the file to be modified directly by right clicking on the icon, choosing the graphic editor MTPaint 3.40 in the mask. Each time, an error appears ("can not open the file"), although - giving OK on that message - it then manages to access that application.
Then, to open the image file that I intend to change, I must necessarily click on the "upload image" icon.
The problem does not occur in Lubuntu 16.4 instead.
Result from terminal
(mtpaint:8824): Gdk-WARNING **: 15:59:42.823: locale not supported by C library
(mtpaint:8824): GLib-GIO-CRITICAL **: 15:59:43.135: g_dbus_proxy_new: assertion 'G_IS_DBUS_CONNECTION (connection)' failed