I have tried looking in the various documentation for ART including video examples and forums questions for an answer to my issue with no success so I am asking here. I am starting to use the AI SMART masking and so far am able to create a mask and then tell, for example, “Color/Tone Correction” to use that generated PNG file mask as an “External mask”. So far so good. But if I decide I no longer want to use any “External mask” I know that I can disable “External mask” yet I can’t find a way to delete/clear the filename I had previously used for the “External mask”. I know I can edit the .arp file to delete it but is there a way in the ART GUI to delete/clear the filename? If I am missing something obvious it would not be the first time 
I did not find a way to remove just the file name. But you can click on the reset icon in the Mask Parameters list to clear the name. If it is not what you want maybe you can ask Alberto on GitHub?