Fixed retrieval of non existing values in getSingleReferenceValue

Author: sjahreis

kotlin-insight-client/kotlin-insight-client-api/src/main/kotlin/com/linkedplanet/kotlininsightclient/api/model/Model.kt

@@ -153,7 +153,7 @@ fun InsightObject.getDateTimeValue(id: Int): ZonedDateTime? =
 fun InsightObject.getSingleReferenceValue(id: Int): InsightReference? =
     getAttribute(id)
         ?.value
-        ?.single()
+        ?.firstOrNull()
         ?.referencedObject
         ?.let {
             InsightReference(