fix: RangeError when un-nesting certain blocks

[matthewlipski]

This PR fixes an error caused when trying to un-nest blocks which have children.

Closes #481 Closes #598

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[sb8244]

May be unrelated, I'm attempting to use this function to fix liftListItem in a function I'm creating.

The result when calling it is that all indentation is removed. So instead of going up 1 level, it goes to the root.

It may be a side effect of the function I'm writing (trying to have sinkListItem only affect the single node that's selected) though.