I think this is only possible if StudioOne is calling the IPlugView::checkSizeConstraint after calling IPlugView::removed. The docs for removed say:
You have to remove all your own views from the parent window or view.
I’m not sure that it makes sense for the host to call checkSizeConstraint in this case, because the host should expect that the editor isn’t displaying anything useful at this point.
Therefore, I don’t think this is a JUCE bug - the assertion is useful to point out that something a bit unexpected has happened.