fix(typescript-fetch): filter Null type from oneOf to prevent missing model references

In OpenAPI 3.1, oneOf can include type: 'null' to represent nullable
unions. The codegen maps this to a "Null" model name and adds it to the
oneOf variants list and imports. However, no Null.ts model file is
generated, causing TypeScript import errors like:
  Module '"../models/index"' has no exported member 'Null'

Fix: remove "Null" from cm.oneOf and filter it from oneOfModels during
model post-processing, following the same pattern used by the C#, Java,
and PowerShell generators. The model is marked as nullable instead.

Author: guizmaii

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/TypeScriptFetchClientCodegen.java

@@ -813,12 +813,21 @@ private ExtendedCodegenModel processCodeGenModel(ExtendedCodegenModel cm) {
                 .map(CodegenComposedSchemas::getOneOf)
                 .orElse(Collections.emptyList());
 
+        // Remove "Null" from oneOf variants. In OpenAPI 3.1, oneOf can include
+        // `type: 'null'` to represent nullable types. The codegen maps this to a
+        // "Null" model name, but no Null model file is generated, causing import
+        // errors. Instead, mark the model as nullable and filter out the Null entry.
+        if (cm.oneOf != null && !cm.oneOf.isEmpty() && cm.oneOf.remove("Null")) {
+            cm.isNullable = true;
+        }
+
         // create a set of any non-primitive, non-array types used in the oneOf schemas which will
         // need to be imported.
         cm.oneOfModels = oneOfsList.stream()
                 .filter(cp -> !cp.getIsPrimitiveType() && !cp.getIsArray())
                 .map(CodegenProperty::getBaseType)
                 .filter(Objects::nonNull)
+                .filter(baseType -> !"Null".equals(baseType))
                 .collect(Collectors.toCollection(TreeSet::new));
 
         // create a set of any complex, inner types used by arrays in the oneOf schema (e.g. if

modules/openapi-generator/src/test/java/org/openapitools/codegen/typescript/fetch/TypeScriptFetchClientCodegenTest.java

@@ -444,6 +444,31 @@ public void testOneOfModelsImportNonPrimitiveTypes() throws IOException {
         TestUtils.assertFileContains(testResponse, "import type { OptionThree } from './OptionThree'");
     }
 
+    @Test(description = "Verify nullable oneOf does not generate Null model references")
+    public void testNullableOneOfDoesNotImportNullModel() throws IOException {
+        File output = generate(
+            Collections.emptyMap(),
+            "src/test/resources/3_0/typescript-fetch/nullable-oneOf.yaml"
+        );
+
+        Path nullableResult = Paths.get(output + "/models/NullableResult.ts");
+        TestUtils.assertFileExists(nullableResult);
+
+        // Should not import or reference a non-existent "Null" model
+        TestUtils.assertFileNotContains(nullableResult, "import type { Null }");
+        TestUtils.assertFileNotContains(nullableResult, "NullFromJSON");
+        TestUtils.assertFileNotContains(nullableResult, "NullToJSON");
+        TestUtils.assertFileNotContains(nullableResult, "instanceOfNull");
+        // Should contain the valid model types
+        TestUtils.assertFileContains(nullableResult, "FileLocation");
+        TestUtils.assertFileContains(nullableResult, "DetailedLocation");
+        // Union type should not include Null
+        TestUtils.assertFileContains(nullableResult, "export type NullableResult = DetailedLocation | FileLocation");
+
+        // No Null.ts model file should be generated
+        TestUtils.assertFileNotExists(Paths.get(output + "/models/Null.ts"));
+    }
+
     @Test(description = "Verify validationAttributes works with withoutRuntimeChecks=true")
     public void testValidationAttributesWithWithoutRuntimeChecks() throws IOException {
         Map<String, Object> properties = new HashMap<>();

modules/openapi-generator/src/test/resources/3_0/typescript-fetch/nullable-oneOf.yaml

@@ -0,0 +1,45 @@
+openapi: 3.1.0
+info:
+  version: 1.0.0
+  title: testing nullable oneOf (Null type filtering)
+paths:
+  /locations/{locationId}:
+    get:
+      operationId: getLocation
+      parameters:
+        - name: locationId
+          in: path
+          required: true
+          schema:
+            type: string
+      responses:
+        200:
+          description: OK
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/NullableResult'
+components:
+  schemas:
+    NullableResult:
+      oneOf:
+        - $ref: '#/components/schemas/FileLocation'
+        - $ref: '#/components/schemas/DetailedLocation'
+        - type: 'null'
+    FileLocation:
+      type: object
+      properties:
+        path:
+          type: string
+      required:
+        - path
+    DetailedLocation:
+      type: object
+      properties:
+        path:
+          type: string
+        size:
+          type: integer
+      required:
+        - path
+        - size