Once you have added something it is there to stay.
What I do is make the level save it, add onething run it ,if it works fine I save it.
And just keep doing it that way. say you add a crate and it gives an error or you don't think that is the crate you want in the game you can quit the editor and when it ask to save the game say no and then restart that level and all the stuff you didn't want or gave erros are now gone.
It will take a little more time to do your game but you won't end up with a bunch of stuff you don't want in the game and you won't have to start over like your going to do now with the game your working on.
