Fix Dart native nullable number deserialization (#20238)

modules/openapi-generator/src/main/resources/dart2/serialization/native/native_class.mustache

@@ -361,7 +361,9 @@ class {{{classname}}} {
                 {{^vendorExtensions.x-is-optional}}
         {{{name}}}: {{#isNullable}}json[r'{{{baseName}}}'] == null
             ? {{#defaultValue}}{{{.}}}{{/defaultValue}}{{^defaultValue}}null{{/defaultValue}}
-            : {{/isNullable}}{{{datatypeWithEnum}}}.parse('${json[r'{{{baseName}}}']}'),
+            : {{/isNullable}}{{^isNullable}}{{^required}}json[r'{{{baseName}}}'] == null
+            ? {{#defaultValue}}{{{.}}}{{/defaultValue}}{{^defaultValue}}null{{/defaultValue}}
+            : {{/required}}{{/isNullable}}{{{datatypeWithEnum}}}.parse('${json[r'{{{baseName}}}']}'),
                 {{/vendorExtensions.x-is-optional}}
               {{/isNumber}}
               {{^isNumber}}

modules/openapi-generator/src/test/java/org/openapitools/codegen/dart/DartClientCodegenTest.java

@@ -170,6 +170,22 @@ public void testRequiredNullableFieldsDoNotAssertNonNull() throws Exception {
                 "json[r'nickname'] != null");
     }
 
+    @Test(description = "Optional numeric fields should guard against null values before parsing")
+    public void testOptionalNumericFieldsGuardAgainstNull() throws Exception {
+        List<File> files = generateDartNativeFromSpec(
+                "src/test/resources/3_0/dart/dart-native-deserialization-bugs.yaml");
+
+        File modelFile = files.stream()
+                .filter(f -> f.getName().equals("optional_number_model.dart"))
+                .findFirst()
+                .orElseThrow(() -> new AssertionError("optional_number_model.dart not found in generated files"));
+
+        TestUtils.assertFileContains(modelFile.toPath(),
+                "problem: json[r'problem'] == null");
+        TestUtils.assertFileContains(modelFile.toPath(),
+                ": num.parse('${json[r'problem']}')");
+    }
+
     @Test(description = "Nullable nested arrays of complex types should preserve null entries")
     public void testNullableNestedComplexArraysPreserveNullEntries() throws Exception {
         List<File> files = generateDartNativeFromSpec(

modules/openapi-generator/src/test/resources/3_0/dart/dart-native-deserialization-bugs.yaml

@@ -32,6 +32,11 @@ components:
         nickname:
           type: string
           nullable: true
+    OptionalNumberModel:
+      type: object
+      properties:
+        problem:
+          type: number
     ComplexNestedArrayNullableModel:
       type: object
       properties: