Thank you very much for the error log you provided! It seems like the error is happening in the actionlog plugin, which is new in Joomla 3.9, and the error occurs when one of our really old modules or plugins are uninstalled. We're still investigating as we can't recreate it (yet), but you've really helped us narrow down where to look.
For background... Long ago, JFBConnect had a lot of individual modules for thinks like Facebook Like, Facebook Fan, Facebook Recommendations, etc. Additionally, we had individual update servers URLs for each of our modules and plugins. In our install script, we try to remove a lot of these old extensions that are no longer used. That process, eventually, gets to the action log plugin, if enabled, which seems to throw out that "Error get() on undefined" message.
Again, we can't recreate it, but we can see where the code goes to that line. We're just unsure why, sometimes, that error is thrown.
I'd recommend, temporarily, disabling the Action Log - Joomla plugin and then reinstall. That should have no errors.. and would also confirm our suspicions.
In investigating, and from previous user reporting, using JFBConnect after that error doesn't have problems. It's pretty much the last step we take to clean things up, so everything is installed and configured at that point.. but obviously, a big error like that is scary.
Please test with the actionlog plugin disabled and let us know how that goes and, once again, thank you so much for that log!!
Alex