Python controllers aren't registered fast enough

[rchiodo]

Testing microsoft/vscode#127378

Opened IW with a python kernel
Reloaded window

Got this as my active kernel in the IW that was opened:

After a minute or so it switched to the correct kernel

[rebornix]

@joyceerhl what I'm seeing is the jupyter extension doesn't contribute kernels for the iw restored.

[joyceerhl]

@rchiodo is that Julia kernel contributed by the Julia extension or by us? Are there any others in the list? I'm wondering if this is because we're rerunning our kernel discovery to reregister controllers, so the correct controller is initially unavailable.

[rchiodo]

It's a real kernel, so the julia kernel is contributed by us. And yes I suspect the julia kernel comes back first and the python one will come later. Don's work to have a placeholder kernel should solve this.

[joyceerhl]

This problem might also be alleviated by #6422 I take that back, since the Julia kernel is contributed by us, we must've already been activated