[analyzer] fix an npe in fix_internal.dart:2171

Bug: #39673
Change-Id: Iad903410ba5fecfa61992c2dfba9289c07234269
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/127422
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Devon Carew <devoncarew@google.com>

Author: devoncarew

pkg/analysis_server/lib/src/services/correction/fix_internal.dart

@@ -2168,6 +2168,9 @@ class FixProcessor extends BaseProcessor {
     // prepare target declaration
     var targetDeclarationResult =
         await sessionHelper.getElementDeclaration(targetElement);
+    if (targetDeclarationResult == null) {
+      return;
+    }
     if (targetDeclarationResult.node is! ClassOrMixinDeclaration &&
         targetDeclarationResult.node is! ExtensionDeclaration) {
       return;