Check if reference field exists in the processed entity when migrating references

Thanks for your effort on the D8 branch.

Problem/Motivation

I tried to test the module in Drupal 8. I encountered a problem when merging the terms. I get the following error.

Error: Call to a member function getValue() on null in Drupal\term_merge\TermMerger->migrateReference() (line 174 of modules/contrib/term_merge/src/TermMerger.php).

While investigating the issue I saw, that it tries to get the value of a field, that doesn't exist for currently processed entity. The reason is, that not every entity reference field to term exists for each node bundle. It becomes quite obvious when having a look into the actual code.

$referenceFieldNames = $this->findTermReferenceFieldNames();
$referencingEntities = $this->loadReferencingEntities($fromTerm);

foreach ($referencingEntities as $entity) {
  foreach ($referenceFieldNames as $fieldName) {
    $values = $entity->{$fieldName}->getValue();

Proposed resolution

Do a proper check, if the field exists, before trying to get the value.