I have been trying to make a Safari Extension the last days, and I have to say that this is a nightmarish and unpredictable experience.
I have launched the app first to register with Safari. The extension then shows up in my regular Safari extension and it seems quite stable, but debugging doesn't work.
When I launch the extension and run Safari, a new instance is started, so I then have to enable unsigned extensions every time I make a change. The plugin is often not injected. And if it is, it crashes shortly after.
There is no feedback on why any of this hapens.
Am I doing something wrong, am I just unlucky, or is this part of OS X development simply not ready for developers?
It's hard to say without seeing exactly what's going on on your machine.
Can you get a screen recording of this behavior and then attempt to get a sysdiagnose? Please file that through Feedback Assistant and post the number on here and I will follow up.