Guard against missing token (issue 27963)

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 3e3da6be9d60e47bc3a153d06d7009938ff90f08..aaa21fa30a6033034eab4053a7d54c4d21667bb4 100644
--- a/pkg/analysis_server/lib/src/services/correction/fix_internal.dart
+++ b/pkg/analysis_server/lib/src/services/correction/fix_internal.dart
@@ -489,9 +489,17 @@ class FixProcessor {
           AstNode targetNode = targetElement.computeNode();
           if (targetNode is FunctionDeclaration) {
             FunctionExpression function = targetNode.functionExpression;
-            targetOffset = function.parameters.leftParenthesis.end;
+            Token paren = function.parameters.leftParenthesis;
+            if (paren == null) {
+              return;
+            }
+            targetOffset = paren.end;
           } else if (targetNode is MethodDeclaration) {
-            targetOffset = targetNode.parameters.leftParenthesis.end;
+            Token paren = targetNode.parameters.leftParenthesis;
+            if (paren == null) {
+              return;
+            }
+            targetOffset = paren.end;
           } else {
             return;
           }