Guard against NPE caused by invalid code (issue 28829)

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

Index: pkg/analysis_server/lib/src/computer/computer_hover.dart
diff --git a/pkg/analysis_server/lib/src/computer/computer_hover.dart b/pkg/analysis_server/lib/src/computer/computer_hover.dart
index 5648d886fc9379776e43c7d72a088c14fc077a9e..f718bc2a9730da4ef23ee4bef7bbe2175547e36b 100644
--- a/pkg/analysis_server/lib/src/computer/computer_hover.dart
+++ b/pkg/analysis_server/lib/src/computer/computer_hover.dart
@@ -114,6 +114,11 @@ class DartUnitHoverComputer {
     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);