Guard against an AST node with no end token (issue 30089)

pkg/analyzer_plugin/lib/src/utilities/completion/completion_target.dart

Index: pkg/analyzer_plugin/lib/src/utilities/completion/completion_target.dart
diff --git a/pkg/analyzer_plugin/lib/src/utilities/completion/completion_target.dart b/pkg/analyzer_plugin/lib/src/utilities/completion/completion_target.dart
index b825a8447bd49e569c6d662096989d710dfaba58..431384112e3111404ab4fa9c32e5cab54f8c8d73 100644
--- a/pkg/analyzer_plugin/lib/src/utilities/completion/completion_target.dart
+++ b/pkg/analyzer_plugin/lib/src/utilities/completion/completion_target.dart
@@ -451,6 +451,9 @@ class CompletionTarget {
    * [CompletionTarget] associated with the given [offset].
    */
   static bool _isCandidateToken(Token token, int offset) {
+    if (token == null) {
+      return false;
+    }
     // A token is considered a candidate entity if the cursor offset is (a)
     // before the start of the token, (b) within the token, (c) at the end of
     // the token and the token is a keyword or identifier, or (d) at the