Foreign Keys causing issues with Many to One relationship in Spring. Null Pointer error - spring

Question

We have a User Entity and an Activity Entity where we have a Many to one relationship with User and a One to Many relationships with Activity. We keep getting a null pointer error every time we try to post something. Basically, When I try to add a new activity with an associated foreign key int(for the user). It gives me this message. Don't know why it has a null pointer exception Error
This is what my Activity Controller looks like:
#RestController
public class Activitiy_Contoller {
#Autowired
private User_Service userService;
private Activity_Service activityService;
#RequestMapping(method=RequestMethod.GET, value="/getactivity")
public List<Activity_Entity> getAllActivity()
{
return Activity_Service.getAllActivity();
}
#GetMapping(value="/getactivityfromid/{id}")
public Activity_Entity getActivity(#PathVariable int id)
{
Activity_Entity temp = activityService.getActivity(id);
return temp;
}
#RequestMapping(method=RequestMethod.POST, value="/addActivity")
public void addActivity(#RequestBody Activity_Entity activity)
{
activityService.addActivity(activity);
}
#DeleteMapping(value="/deleteActivity/{id}")
public void deleteActivity(#PathVariable int id)
{
activityService.deleteActivity(id);
}
}
My Relationship in Activity Entity looks like this:
#Entity
#Table(name ="activity_type")
public class Activity_Entity {
//-----------Values-------------
#Id
#Column(name="Id")
#GeneratedValue(strategy=GenerationType.AUTO)
public int id;
//can get the activities for one user with
//this value using the activity_repo
#JsonIgnore
#ManyToOne
#JoinColumn(name ="user_id")
public User_Entity user;
My Relationship in User Entity looks like this:
#Entity
public class User_Entity {
// ----------------Values------------------
#Id
#GeneratedValue(strategy=GenerationType.AUTO)
public int Userid;
//Many groups to one user
//so can get all groups for one user
//from this value
#OneToMany(mappedBy = "user")
Set<Activity_Entity> Activities;
private Set<Group_Entity> Groups;