If noting selected: Call to a member function uuid() on null in Drupal\media_library\MediaLibraryEditorOpener->getSelectionResponse()

This MR keeps the widget open and shows the given error message inside the widget so that the user knows what's going on.

What I can't sort out is the same error message displayed once again after the next page reload. It is set by viewsFormValidate within the same class, but the MR does a $form_state->clearErrors() but that still doesn't avoid that extra message.

I know, this also needs some tests, but I won't be able to provide any short term. I just wanted to provide the fix as it happens frequently in a customer project.

Can we add a small assertion somewhere for the change.

Thanks.