Guard against NPE caused by invalid code (issue 28829)

pkg/analysis_server/lib/src/computer/computer_hover.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library computer.hover;
 
 import 'package:analysis_server/plugin/protocol/protocol.dart'
     show HoverInformation;
 import 'package:analysis_server/src/computer/computer_overrides.dart';
 import 'package:analysis_server/src/utilities/documentation.dart';
@@ skipping 96 matching lines @@
     return null;
   }
 
   String _computeDocumentation(Element element) {
     if (element is FieldFormalParameterElement) {
       element = (element as FieldFormalParameterElement).field;
     }
     if (element is ParameterElement) {
       element = element.enclosingElement;
     }
+    if (element == null) {
+      // This can happen when the code is invalid, such as having a field formal
+      // parameter for a field that does not exist.
+      return null;
+    }
     // The documentation of the element itself.
     if (element.documentationComment != null) {
       return removeDartDocDelimiters(element.documentationComment);
     }
     // Look for documentation comments of overridden members.
     OverriddenElements overridden = findOverriddenElements(element);
     for (Element superElement in []
       ..addAll(overridden.superElements)
       ..addAll(overridden.interfaceElements)) {
       String rawDoc = superElement.documentationComment;
       if (rawDoc != null) {
         Element interfaceClass = superElement.enclosingElement;
         return removeDartDocDelimiters(rawDoc) +
             '\n\nCopied from `${interfaceClass.displayName}`.';
       }
     }
     return null;
   }
 
   static _safeToString(obj) => obj?.toString();
 }