Type 'string | null' is not assignable to type 'string'. Type 'null' is not assignable to type 'string'

Error: Type 'string | null' is not assignable to type 'string'. Type 'null' is not assignable to type 'string'. Error : Type '{ id?: string | undefined; title?: string | undefined; content?: string | undefined; }' is not assignable to type 'Post'. Types of property 'id' are incompatible. Type 'string | undefined' is not assignable to type 'string'. Type 'undefined' is not assignable to type 'string'.

export interface Post{
    id:string;
    title:string;
    content:string;
}

import { Component,OnInit} from '@angular/core';
import { NgForm } from '@angular/forms';
import { ActivatedRoute, ParamMap } from '@angular/router';
import { Post } from '../post.model';

import { PostsService } from '../posts.service';

@Component({
  selector: 'app-post-create',
  templateUrl: './post-create.component.html',
  styleUrls: ['./post-create.component.css']
})
export class PostCreateComponent implements OnInit {

// newPost='No Content'
enteredTitle='' 
  enteredContent=''
private mode='create'
private postId:string 
private post:Post
  // postId: string | null | undefined;

constructor(public postsService:PostsService
  ,public route: ActivatedRoute){}

ngOnInit(){
  this.route.paramMap.subscribe((paramMap:ParamMap)=>{
    if(paramMap.has('postId')){
      this.mode='edit'
      this.postId=paramMap.get('postId')
      this.post=this.postsService.getPost(this.postId)
    }else{
      this.mode='create'
      this.postId="null";
    }
  });
}

onAddPost(form:NgForm){ 
  if(form.invalid)
  {
    return
  } 
  console.log((form.value.title,form.value.content));
  this.postsService.addPost(form.value.title,form.value.content)

form.resetForm();

}
}