Fix Composites and Sub-Trees not ending their Children when exited

[achynes]

(Plus a couple of null deref fixes.)

I was having issues when I first set up this plugin on my work project; various stateful things were hanging around unexpectedly. I realised this was missing, and once added, everything has worked well with it for the past 8 months (the game is in Early Access: https://store.steampowered.com/app/1724030/Eyes_of_Hellfire/).

I couldn't get those random changed 'using' directives to not show as modified, whatever I tried to do with line endings in VS and git, so hopefully not an issue!

And thanks for the super helpful plugin! It was just the thing I needed to stand up a bunch of AI behaviours with minimal code.