I get this error when trying to open the diff ("review changes") panel in the save dialog:
Uncaught TypeError: Cannot read property 'getData' of null at AddLinkSaveDialog.ve.ui.MWSaveDialog.updateReviewMode
The references line is
diffMode = this.reviewModeButtonSelect.findSelectedItem().getData(),
so this caused by neither the visual diff mode nor the source diff mode button being selected. Probably some side effect of AddLinkSaveDialog, even though we don't intentionally affect the diff buttons there.