dev: "This value should not be null" for optional fields

I have debugged this problem together with a colleague, as we also want to use conflict together with scheduler.

We are pretty sure that this added line of code causes the problem:
https://git.drupalcode.org/project/conflict/-/commit/d1958eca3852ef55c87...

The key/value pair conflictWidget will be added to the value array of all fields. Later on the validation of the entity is triggered. Because of the additional information in the value (here data) array, the array is not empty and is considered complex (see [1]). For this reason the array entries are validated individually. Because the datetime information is still null, the NotNullValidator reports a validation.

Another negative side effect is, that the comparison of the local node and the server node in the conflict module always leads to the result "has changes". FieldComparatorDefault is the only available comparator, which does a simple equals comparison. This comparison is always false, because there is this additional entry conflictWidget.

Proposed solution:

A) Place the conflictWidget information at a different location

B) Add a tidy up function, which is called, when conflict finished its job. We did a quick and dirty try at the end of \Drupal\conflict\Entity\ContentEntityConflictHandler::entityFormEntityBuilder, which seem to fix the bug.

foreach (array_keys($form_display->getComponents()) as $field_name) {
  if ($entity->getFieldDefinition($field_name)) {
    unset($entity->get($field_name)->conflictWidget);
  }
}

We are working on a patch, which we will provide soon.

[1] \Drupal\Core\TypedData\Validation\RecursiveContextualValidator::validateNode L.163

It turned out that the quick and dirty solution is also a good and clean solution. Here is a patch for conflict:2.0-alpha3

I confirm the patch in comment #6 works for us.

The patch works. Thank you.