function _showEditor(document) {
        // Hide whatever was visible before
        if (!_currentEditor) {
            $("#not-editor").css("display", "none");
        } else {
            _currentEditor.setVisible(false);
            _destroyEditorIfUnneeded(_currentEditorsDocument);
        }
        
        // Ensure a main editor exists for this document to show in the UI
        if (!document._masterEditor) {
            // Editor doesn't exist: populate a new Editor with the text
            _createFullEditorForDocument(document);
        }
        
        _doShow(document);
    }
Make the given document's editor visible in the UI, hiding whatever was
visible before. Creates a new editor if none is assigned.
@param {!Document} document