Problem: SKOS import fatals with "Call to a member function getValue() on null" when skos:related points at a concept that was not imported #2417

$concepts = $this->getRootConcepts();
foreach ($concepts as $item) {
    ... 
    $this->addConcept($item, $this->parent->id);
}   

addConcept() sets the stamp and recurses through skos:narrower only:

$concept->set('atom:id', (int) $term->id);

foreach ($concept->allResources('skos:narrower') as $item) {
    $this->addConcept($item, $term->id);
}   

Pass two walks every concept in the graph and dereferences that stamp on both ends of every skos:related, unguarded (line 113):

foreach ($this->graph->allOfType('skos:Concept') as $c) {
    foreach ($c->allResources('skos:related') as $r) {
        $relations->insert($c->get('atom:id')->getValue(), $r->get('atom:id')->getValue());
    }   
}   

Pass one visits only what is reachable from a root via skos:narrower; pass two visits everything. Any concept in the second set but not the first has no atom:id, get() returns null, and getValue() is a fatal.

getRootConcepts() is what makes this easy to hit:

$conceptScheme = $this->graph->get('skos:ConceptScheme', '^rdf:type');
if (null !== $conceptScheme) {
    $topConcepts = $conceptScheme->allResources('skos:hasTopConcept');
    if (0 < count($topConcepts)) {
        return $topConcepts;
    }   
}   

return $this->graph->allOfType('skos:Concept');

### Expected Behavior

1. Save this as sample.ttl:

turtle
@prefix skos: <http://www.w3.org/2004/02/skos/core#> .
@prefix ex:   <http://example.org/> .

ex:scheme a skos:ConceptScheme ;
    skos:hasTopConcept ex:a .
    
ex:a a skos:Concept ;
    skos:prefLabel "Imported"@en ;
    skos:related ex:b .
    
ex:b a skos:Concept ;
    skos:prefLabel "Never imported"@en .
    
2. Import it into any taxonomy (Manage > Taxonomies > Import, or the XML import).
3. ex:a is created, then the job ends in Error.

ex:b is a skos:Concept, so pass two sees it; it is neither a top concept nor skos:narrower of one, so pass one never stamps it.

Job report from the original report (a 10-concept Places file):

[info] Importing SKOS file: sample_skos.ttl
[info] Type of scheme: file 
[info] Taxonomy: Places
[info] Term ID: 110
[info] The graph contains 10 concepts.
[err]  Exception: Call to a member function getValue() on null
[err]  File: plugins/sfSkosPlugin/lib/sfSkosPlugin.class.php
[err]  Line: 113

**Expected Behavior**

```markdown
The import completes, creating the concepts it can and skipping only the relation it cannot express — with a line in the job log naming the concepts involved, so the cataloguer can see what was dropped and why.

A `skos:related` naming a concept outside the imported set is ordinary, valid SKOS. It should not be able to end the run.

Two things make the current behaviour worse than a simple failure:

- **The partial import persists.** Terms created before the fatal are already saved, so a re-run duplicates them and the file cannot simply be re-imported after a fix.
- **The error names no concept.** It reports a file and a line, so there is nothing to point the cataloguer at in a 10-concept file, let alone a large vocabulary. 

### Possible Solution

```php
foreach ($this->graph->allOfType('skos:Concept') as $c) {
    foreach ($c->allResources('skos:related') as $r) {
        $subject = $c->get('atom:id');
        $object = $r->get('atom:id');
        
        if (null === $subject || null === $object) {
            $this->logger->info($this->i18n->__(
                'Skipping skos:related between %1% and %2%: one of them was not imported.',
                ['%1%' => $c->getUri(), '%2%' => $r->getUri()]
            )); 
            
            continue;
        }   
        
        $relations->insert($subject->getValue(), $object->getValue());
    }   
}   

### Context and Notes

AtoM 2.10.1 and 2.10.2 (plugins/sfSkosPlugin/lib/sfSkosPlugin.class.php:113, unchanged between the two). PHP 8.3, MySQL 8, nginx, Ubuntu 22.04.



### Version used

2.10.1

### Operating System and version

22.04

### Default installation culture

en

### PHP version

8.3

### Contact details

johan@theahg.co.za