Simplify method (#873)

idea/idea-analysis/src/org/jetbrains/kotlin/idea/references/KtReference.kt

@@ -47,11 +47,7 @@ abstract class AbstractKtReference<T : KtElement>(element: T)
     }
 
     override fun resolve(): PsiElement? {
-        val psiElements = resolveToPsiElements()
-        if (psiElements.size == 1) {
-            return psiElements.iterator().next()
-        }
-        return null
+        return resolveToPsiElements().singleOrNull()
     }
 
     override fun isReferenceTo(element: PsiElement?): Boolean {