#1475 Check derivative_term instead of source_term (#766)

Co-authored-by: Jonathan Hunt <git@huntdesign.co.nz>

src/Plugin/Action/AbstractGenerateDerivative.php

@@ -160,7 +160,7 @@ class AbstractGenerateDerivative extends EmitEvent {
     // Find the term for the derivative and use it to set the destination url
     // in the data array.
     $derivative_term = $this->utils->getTermForUri($this->configuration['derivative_term_uri']);
-    if (!$source_term) {
+    if (!$derivative_term) {
       throw new \RuntimeException("Could not locate derivative term with uri" . $this->configuration['derivative_term_uri'], 500);
     }