Don't assume we need a constructor too early in build process (#1049)

Fixes #1038

Author: kevmoo

json_serializable/lib/src/utils.dart

@@ -84,10 +84,12 @@ ClassConfig mergeConfig(
   assert(config.ctorParamDefaults.isEmpty);
 
   final constructor = annotation.constructor ?? config.constructor;
-  final constructorInstance = constructorByName(classElement, constructor);
+  final constructorInstance =
+      constructorByNameOrNull(classElement, constructor);
 
-  final paramDefaultValueMap = Map<String, String>.fromEntries(
-      constructorInstance.parameters
+  final paramDefaultValueMap = constructorInstance == null
+      ? <String, String>{}
+      : Map<String, String>.fromEntries(constructorInstance.parameters
           .where((element) => element.hasDefaultValue)
           .map((e) => MapEntry(e.name, e.defaultValueCode!)));
 
@@ -110,6 +112,18 @@ ClassConfig mergeConfig(
   );
 }
 
+ConstructorElement? constructorByNameOrNull(
+  ClassElement classElement,
+  String name,
+) {
+  try {
+    return constructorByName(classElement, name);
+    // ignore: avoid_catching_errors
+  } on InvalidGenerationSourceError {
+    return null;
+  }
+}
+
 ConstructorElement constructorByName(ClassElement classElement, String name) {
   final className = classElement.name;
 

json_serializable/test/json_serializable_test.dart

@@ -79,6 +79,7 @@ const _expectedAnnotatedTests = {
   'InvalidFromFunc2Args',
   'InvalidToFunc2Args',
   'Issue713',
+  'Issue1038RegressionTest',
   'JsonConvertOnField',
   'JsonConverterCtorParams',
   'JsonConverterDuplicateAnnotations',

json_serializable/test/src/_json_serializable_test_input.dart

@@ -561,3 +561,21 @@ class ExtraParamToJson {
 
   Map<String, dynamic> toJson(int bob) => throw UnimplementedError();
 }
+
+@ShouldGenerate(r'''
+Map<String, dynamic> _$Issue1038RegressionTestToJson(
+        Issue1038RegressionTest instance) =>
+    <String, dynamic>{
+      'id': instance.id,
+      'ean': instance.ean,
+    };
+''')
+@JsonSerializable(createFactory: false)
+class Issue1038RegressionTest {
+  final String? id;
+  final String? ean;
+
+  Issue1038RegressionTest.id(this.id) : ean = null;
+
+  Issue1038RegressionTest.ean(this.ean) : id = null;
+}