Issue #3632085: Layout "Change" button throws TypeError with Better Exposed Filters

updateRegions() is a #submit handler, so it receives the complete form, and passed it to DisplayPluginBase::submitOptionsForm(). The display expects only the options subform, as Views UI Display::submitForm() passes it; given the complete form the exposed form plugin received NULL, and Better Exposed Filters failed building its widget subforms.

Closes #3632085