Guard against FunctionExpression nodes with null parameters.

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

Index: pkg/analysis_server/lib/src/services/correction/fix_internal.dart
diff --git a/pkg/analysis_server/lib/src/services/correction/fix_internal.dart b/pkg/analysis_server/lib/src/services/correction/fix_internal.dart
index dc59c71f0942701a26a4b8c2aa8a029b73cfb217..dc28bed7ba55bcb9917478de9e8180b5c8a83b3e 100644
--- a/pkg/analysis_server/lib/src/services/correction/fix_internal.dart
+++ b/pkg/analysis_server/lib/src/services/correction/fix_internal.dart
@@ -487,13 +487,13 @@ class FixProcessor {
           AstNode targetNode = targetElement.computeNode();
           if (targetNode is FunctionDeclaration) {
             FunctionExpression function = targetNode.functionExpression;
-            Token paren = function.parameters.leftParenthesis;
+            Token paren = function.parameters?.leftParenthesis;
             if (paren == null) {
               return;
             }
             targetOffset = paren.end;
           } else if (targetNode is MethodDeclaration) {
-            Token paren = targetNode.parameters.leftParenthesis;
+            Token paren = targetNode.parameters?.leftParenthesis;
             if (paren == null) {
               return;
             }