Fix crash when adding scenes with a group to the level scene

[jamie-pate]

Fixes #94274

[KoBeWi]

It does not fix the crash, because you did not delete the _update_groups() call...

Also you can just do an early return at the beginning:

if (!scene_root_node) {
	return;
}

[jamie-pate]

Oops, messed up which lines to stage when committing, that extra line wasn't there when I was testing it obviously :D

[AThousandShips]

Please squash your commits into one, see here

[jamie-pate]

I was going to do that, then I read that you can just set github to do it for you on all PRs.. so I'd hoped that was the case

[Calinou]

I was going to do that, then I read that you can just set github to do it for you on all PRs.. so I'd hoped that was the case

We don't use GitHub's squash and merge feature on the main Godot repository, since it loses the merge commit in the process. We prefer keeping the merge commit to make the Git history more self-documented (even if that makes it longer when not using the --no-merges option).

[akien-mga]

Thanks!