Nest JS: Object is possibly 'undefined' in GraphQL

I tried to make the **subscription update course** in the nest js, but it turns out error **Object is possibly 'undefined**' in this line of code `const course = await this.courseService.findOne({ id: variables.courseId }); `below is my course.resolver.ts

```
@Subscription('courseUpdated', {
resolve: (value) => value,
filter: async (payload, variables) => {
const course = await this.courseService.findOne({ id: variables.courseId });
if (!course) {
throw new NotFoundException('Course not found');
}
if (course.creatorId !== variables.userId) {
throw new ForbiddenException('Course not owned by you');
}
return payload.courseId === variables.courseId;
},
})
courseUpdated(
@Args('courseId') courseId: string,
@Args('userId') userId: string,
) {
return pubSub.asyncIterator(`course:${courseId}`);
}
```
Here is the piece of code in my course.graphql

```
type Course {
id: String
title: String
objective: [String!]
description: String!
language: String
category: String!
tone: String!
modality: String!
classSize: String!
topics: [String!]
duration: Int!
durationLesson: Int!
creatorId: String!

outline: CourseOutline
detailedOutline: CourseDetailedOutline

createdAt: DateTime!
updatedAt: DateTime!

audience: Audience!
}

input UpdateCourse {
title: String
objective: [String!]
description: String
language: String
category: String
tone: String
modality: String
classSize: String
topics: [String!]
duration: Int
durationLesson: Int
audience: CreateAudienceInput

outline: UpdateCourseOutline
detailedOutline: UpdateCourseDetailedOutline
}

type Subscription {
courseUpdated(courseId: String!): Course!
}
```
I read a lot on stackoverflow but still couldn't solve it. Yes indeed some of the value is optional, but how to solve it. I also tried the chaining option like this` const course = await this.courseService.findOne({ id: variables.courseId })?.id;` but still does not work

**System:**
```
"graphql-subscriptions": "2.0.0",
"@nestjs/cli": "^10.1.16",
"@nestjs/graphql": "^12.0.8"
```

Start by checking the reported TypeScript diagnostic in course.resolver.ts and comparing the subscription declaration in course.graphql with the shown NestJS code. Confirm whether the issue is reproducible in this repository; done means the undefined error is resolved and the courseUpdated subscription still behaves as intended.

Written by the indexing model from the issue text.