[core] Fix NullPointer when schema is not set in header (OpenAPITools#2133)

Author: jmini

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

@@ -3297,12 +3297,19 @@ protected List<Map<String, Object>> toExamples(Map<String, Object> examples) {
      */
     private void addHeaders(ApiResponse response, List<CodegenProperty> properties) {
         if (response.getHeaders() != null) {
-            for (Map.Entry<String, Header> headers : response.getHeaders().entrySet()) {
-                String description = headers.getValue().getDescription();
+            for (Map.Entry<String, Header> headerEntry : response.getHeaders().entrySet()) {
+                String description = headerEntry.getValue().getDescription();
                 // follow the $ref
-                Header header = ModelUtils.getReferencedHeader(this.openAPI, headers.getValue());
+                Header header = ModelUtils.getReferencedHeader(this.openAPI, headerEntry.getValue());
 
-                CodegenProperty cp = fromProperty(headers.getKey(), header.getSchema());
+                Schema schema;
+                if(header.getSchema() == null) {
+                    LOGGER.warn("No schema defined for Header '" + headerEntry.getKey() +"', using a String schema");
+                    schema = new StringSchema();
+                } else {
+                    schema = header.getSchema();
+                }
+                CodegenProperty cp = fromProperty(headerEntry.getKey(), schema);
                 cp.setDescription(escapeText(description));
                 cp.setUnescapedDescription(description);
                 properties.add(cp);

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

@@ -22,6 +22,7 @@
 import io.swagger.v3.oas.models.Operation;
 import io.swagger.v3.oas.models.PathItem;
 import io.swagger.v3.oas.models.media.*;
+import io.swagger.v3.oas.models.headers.Header;
 import io.swagger.v3.oas.models.parameters.QueryParameter;
 import io.swagger.v3.oas.models.parameters.RequestBody;
 import io.swagger.v3.oas.models.responses.ApiResponse;
@@ -532,6 +533,24 @@ public void testLeadingSlashIsAddedIfMissing() {
         Assert.assertEquals(co2.path, "/some/path");
     }
 
+    @Test
+    public void testResponseWithNoSchemaInHeaders() {
+        OpenAPI openAPI = TestUtils.createOpenAPI();
+        ApiResponse response2XX = new ApiResponse()
+                .description("OK")
+                .addHeaderObject("x-custom-header", new Header()
+                        .description("a custom header")
+                        .style(Header.StyleEnum.SIMPLE));
+        Operation operation1 = new Operation().operationId("op1").responses(new ApiResponses().addApiResponse("2XX", response2XX));
+        openAPI.path("/here", new PathItem().get(operation1));
+        final DefaultCodegen codegen = new DefaultCodegen();
+        codegen.setOpenAPI(openAPI);
+
+        CodegenResponse cr = codegen.fromResponse("2XX", response2XX);
+        Assert.assertNotNull(cr);
+        Assert.assertTrue(cr.hasHeaders);
+    }
+
     private void verifyPersonDiscriminator(CodegenDiscriminator discriminator) {
         CodegenDiscriminator test = new CodegenDiscriminator();
         test.setPropertyName("DollarUnderscoretype");