Default icon for apps that don't have a compatible icon

[DW8Reaper]

Some apps that appear as applets in the panel don't have an icon probably because they aren't in a compatible format/location and the icon isn't found by the panel. These applets display as blank spaces you can see them if you hover your mouse over them:

It would be great if there was a feature that would display a placeholder icon like a ? or maybe just a number for each app without an icon to make it a little more obvious there is something in the panel. For the one in my screenshot it is obvious because of the blank space but there is another one on the far left next to slack that has no discernible visual clues.

This happens for me with the Jebrains toolbox and Duplicity I also have had some other apps do it in the past that I don't quite remember (and don't use frequently)