Fix OpenAPITools#5381

catch NPE cases in preprocessOpenAPI
updated samples

Author: alexsuperdev

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

@@ -710,7 +710,7 @@ public void postProcessParameter(CodegenParameter parameter) {
     //override with any special handling of the entire OpenAPI spec document
     @SuppressWarnings("unused")
     public void preprocessOpenAPI(OpenAPI openAPI) {
-        if (useOneOfInterfaces) {
+        if (useOneOfInterfaces && openAPI.getComponents() != null) {
             // we process the openapi schema here to find oneOf schemas and create interface models for them
             Map<String, Schema> schemas = new HashMap<String, Schema>(openAPI.getComponents().getSchemas());
             if (schemas == null) {
@@ -733,11 +733,13 @@ public void preprocessOpenAPI(OpenAPI openAPI) {
                             schemas.put(opId, requestSchema);
                         }
                         // process all response bodies
-                        for (Map.Entry<String, ApiResponse> ar : op.getValue().getResponses().entrySet()) {
-                            ApiResponse a = ModelUtils.getReferencedApiResponse(openAPI, ar.getValue());
-                            Schema responseSchema = ModelUtils.getSchemaFromResponse(a);
-                            if (responseSchema != null) {
-                                schemas.put(opId + ar.getKey(), responseSchema);
+                        if (op.getValue().getResponses() != null){
+                            for (Map.Entry<String, ApiResponse> ar : op.getValue().getResponses().entrySet()) {
+                                ApiResponse a = ModelUtils.getReferencedApiResponse(openAPI, ar.getValue());
+                                Schema responseSchema = ModelUtils.getSchemaFromResponse(a);
+                                if (responseSchema != null) {
+                                    schemas.put(opId + ar.getKey(), responseSchema);
+                                }
                             }
                         }
                     }