Create the following WT document:
Run the following commands to delete the 'B' from the reference body:
var sm = ve.init.target.surface.model; sm.change( ve.dm.Transaction.newFromRemoval( sm.documentModel, new ve.Range( 9, 10 ) ) );
Turn on 'pause on uncaught exceptions' then press undo, then redo. Observe that an exception is thrown because the ContentBranchNode has no root.