Component value is not nullable

[grreeenn]

Hello,

Thanks for the great component.
I use it to edit schedules and create new ones. There's no problem on editing, as an expression already exists; but when I create a new schedule, I get an error 'ERROR Error: Error: Expression is empty', because there is no initial expression in this case (and I don't want to add one to avoid the chance of user error). Although everything works, but there's still a red spot in the console :(

I could not determine the cause at the first glance; may be you can guide me where to look for it, so I'll make a PR?