Return 'null' rename refactoring in case of a null element.

pkg/analysis_server/lib/src/services/refactoring/refactoring.dart

Index: pkg/analysis_server/lib/src/services/refactoring/refactoring.dart
diff --git a/pkg/analysis_server/lib/src/services/refactoring/refactoring.dart b/pkg/analysis_server/lib/src/services/refactoring/refactoring.dart
index acf741f1986f37e919fff67df1542cdd0d1cdcaa..a4d3c6b6a334daf1354de3ce8672994dbf3d78c3 100644
--- a/pkg/analysis_server/lib/src/services/refactoring/refactoring.dart
+++ b/pkg/analysis_server/lib/src/services/refactoring/refactoring.dart
@@ -356,6 +356,9 @@ abstract class RenameRefactoring implements Refactoring {
    * type.
    */
   factory RenameRefactoring(SearchEngine searchEngine, Element element) {
+    if (element == null) {
+      return null;
+    }
     if (element is PropertyAccessorElement) {
       element = (element as PropertyAccessorElement).variable;
     }