Well, 8 weeks after the initial reply from @themeisle and 5 weeks since I reached out asking for an update, I finally grew tired of having to “fix” (rollback the plugin) on my client’s site every time they try to update Orbit Fox. I found a fix on my own but it is very hacky and I would not recommend what I did because it is not a fix in any sense of the word.
I realize this site is using an old version of Hestia Pro and that they should keep a current license, but the error is not being caused by the theme; it occurs when updating the **free** plugin that ThemeIsle uses to extend its themes. My client’s site makes use of Orbit Fox features so it is not as simple as just deactivating the plugin; that would cause other issues because functionality would be removed.
It is frustrating that ThemeIsle has ignored this issue, despite releasing 5 minor updates to Orbit Fox since I alerted them to the error. I will remember this poor support if/when ever considering a future purchase from them.
For those like @grossmann who also experienced this error with Hestia Pro 2.0.4, my “fix” was to comment out everything from line 10 to the end of the file in hestia-pro/inc/admin/onboarding/class-themeisle-onboarding.php
I am not concerned this “fix” will break with future Orbit Fox updates, since class-themeisle-onboarding.php appears to be used only for initial setup and importing of a site library. Because the client site does not have an active license, there will not be any theme updates to break anything.
Again, I wouldn’t recommend this fix to anyone, especially if not using Hetia Pro 2.0.4, as I have no idea if this file has changed in recent versions. But, wanted to share my experience so that any others can at least see what worked in my situation.