Attempt to read property "ref_department_id" on null in laravel 8 - laravel

Question

My problem is its hard to get the data from relationship field. There is no null value in table but the error shows attempt to read null value.The data connected with api.Its work when i put if and else but i cant figure out the correct solution for this problem.
Here is my function.php line
function getUnitClerk($unit_id){
$unit = Unit::where('id', $unit_id)->first();
$division = Division::where('id', $unit->ref_division_id)->first();
$department = Department::where('id', $division->ref_department_id)->first();
}
Controller
<?php
namespace App\Http\Controllers;
use Illuminate\Http\Request;
use App\Platform\Bpms;
use Session;
use Illuminate\Support\Facades\DB;
use App\User;
class AdminController extends Controller
{
public function __construct()
{
$this->middleware('guest');
}
public function dashboard()
{
if(!auth()->check())
{
return redirect('login')->with('message', 'Please log in!');
}else{
$roleOfUser = DB::table('role_of_user')->where('user_id','=',auth()->user()->id)->get();
$tasklist = Bpms::queryForTaskListByAssignee(auth()->user()->id);
$list_role = '';
if(count($roleOfUser) > 0){
foreach($roleOfUser AS $roleUser){
if($roleUser != ''){
$grouptasklist = Bpms::queryForTaskList($roleUser->role_name);
$tasklist = array_merge($tasklist, $grouptasklist);
$list_role .= $roleUser->role_name;
$list_role .= ",";
}else{
$tasklist = array();
}
}
}
if(count($tasklist) > 0){
foreach ($tasklist as $i => $task) {
$taskname = $task['taskDefinitionKey'];
$overdue = 'overdue';
if((stripos($taskname, $overdue) !== FALSE)){
$overdue_task2[] = 1;
}else{
$overdue_task2[] = 0;
}
}
$overdue_task = array_sum($overdue_task2);
}else{
$overdue_task = 0;
}
$new = DB::table('history')->where('history_status', '=', 2)->where('history_sender_id','=',auth()->user()->id)->get();
$newtask = count($new);
$inprogress = count($tasklist);
$closed = DB::table('history')
->whereRaw('(history_status = 6)')
->where('created_by','=',auth()->user()->id)
->get();
$closedtask = count($closed);
$lastlogin = DB::table('audit_trail')
->where('adt_action_name', '=', 1)
->where('adt_created_by','=',auth()->user()->id)
->orderBy('adt_created_date', 'desc')
->skip(1)
->take(1)
->first();
// echo '<pre>';
// print_r($tasklist);
// $historyCheck = DB::table('history')->where('history_taskid','=',88989)->exists();
// if($historyCheck)
// {
// $history = DB::table('history')->where('history_taskid','=',88989)->first();
// print_r($history);
// $recordrequest = DB::table('ddms_record_request')->where('ddms_request_id','=',$history->history_request_id)->first();
// print_r($recordrequest);
// exit;
// $requestnumber = DB::table('ddms_request')->where('id','=',$recordrequest->ddms_request_id)->first();
// } else {
// $history = array();
// $recordrequest = array();
// $requestnumber = array();
// }
return view('main.dashboard',compact('tasklist','newtask','inprogress','closedtask', 'overdue_task', 'lastlogin'));
}
}
}
?>
unit model
namespace App\Models\References;
use Illuminate\Database\Eloquent\Model;
class Unit extends Model
{
protected $table = 'ref_unit';
public function division() {
return $this->belongsTo(division::class,'ref_division_id','id');
}
public function section() {
return $this->hasMany(section::class,'ref_unit_id','id');
}
}
Division Model
namespace App\Models\References;
use Illuminate\Database\Eloquent\Model;
class Division extends Model
{
protected $table = 'ref_division';
public function department() {
return $this->belongsTo(department::class,'ref_department_id','id');
}
public function unit() {
return $this->hasMany(unit::class,'ref_division_id','id');
}
}
Department Model
namespace App\Models\References;
use Illuminate\Database\Eloquent\Model;
class Department extends Model
{
protected $table = 'ref_department';
public function tenant() {
return $this->belongsTo('App\Models\References\Tenant','ref_tenant_id','id');
}
public function departmentdetail() {
return $this->hasMany(departmentdetail::class,'ref_department_id','id');
}
public function division() {
return $this->hasMany(division::class,'ref_department_id','id');
}}