TreeMultiSelectField has null value

[mlewis-everley]

I have noticed this for a while and tried to trace the problem (with no luck).

If I am using a TreeMultiSelectField (in getCMSFields) then the value is not set in EditHandler.

The only way I can force this to happen is to overwrite EditHandler::getRecordEditFields() like so:

    private function getRecordEditingFields(DataObject $record)
    {
        $tempForm = Form::create(
            $this,
            'TempEditForm',
            $record->getCMSFields(),
            FieldList::create()
        );

        $tempForm->loadDataFrom($record);
        $fields = $tempForm->Fields();

        $fields = $this->filterRecordEditingFields($fields, $record->ID);

        // Relation field of type TreeMultiSelectField has to have its value forcefully set to a comma
        // seperated srting
        $fields['Categories']->setValue(implode(",", $record->Categories()->column('ID')));

        return $fields;
    }

The only way I can think to change this is maybe look for fields that are instances of TreeMultiSelectField and forcefully set their value?