json_annotation: Fix a potential error with checked: true... (#819)

...when `ArgumentError.message` is `null`

Author: kevmoo

json_annotation/CHANGELOG.md

@@ -1,3 +1,8 @@
+## 4.0.1-dev
+
+- Fix a potential error with `checked: true` when `ArgumentError.message` is 
+  `null`.
+
 ## 4.0.0
 
 - Support null safety.

json_annotation/lib/src/checked_helpers.dart

@@ -104,7 +104,7 @@ class CheckedFromJsonException implements Exception {
         innerStack = null;
 
   CheckedFromJsonException._(
-    this.innerError,
+    Object this.innerError,
     this.innerStack,
     this.map,
     this.key, {
@@ -113,9 +113,12 @@ class CheckedFromJsonException implements Exception {
         badKey = innerError is BadKeyException,
         message = _getMessage(innerError);
 
-  static String? _getMessage(Object? error) {
+  static String _getMessage(Object error) {
     if (error is ArgumentError) {
-      return error.message?.toString();
+      final message = error.message;
+      if (message != null) {
+        return message.toString();
+      }
     }
     if (error is BadKeyException) {
       return error.message;

json_annotation/pubspec.yaml

@@ -1,5 +1,5 @@
 name: json_annotation
-version: 4.0.0
+version: 4.0.1-dev
 description: >-
   Classes and helper functions that support JSON code generation via the
   `json_serializable` package.