Undocking the model views is an awesome new feature I just discovered. However, when you try to toggle the split view (from horizontal to vertical) you get an error. Rather than allowing the user to toggle the undocked views, I recommend that offering to re-dock the models.
Good catch, in a future version we will disable the toggle split when undocked. In the meantime, if you close one of the undocked views, that will also trigger a redock.
Fixed in 2.7.0.