Steps to reproduce

1. have content app 1 with content that links to an other content app 2 (using twinColSelectField)
2. in app 1 link some contents of app 2 and save
3. delete one of the linked contents in app 2
4. open content in app1 again

.. Logs, screenshots, gifs...

Expected results

• content should be editable - editor can fix invalid link
• may be show invalid UUID and allow deleting outdatad link

Actual results

• content cannot be edited, NullPointerException in logs

Workaround

We helped our self by patching com.vaadin.ui.AbstractMultiSelect:

@Override
public void setValue(Set<T> value) {
    Objects.requireNonNull(value);
    // DZ-2133: Fix NullPointerException when trying to edit content with invalid multi value reference (uuid of removed content)
    Set<T> copy = value.stream().filter(Objects::nonNull).collect(Collectors.toCollection(LinkedHashSet::new));

    updateSelection(copy, new LinkedHashSet<>(getSelectedItems()));
} 

There is probably a better place for you to fix it.

Development notes