Trying to access array offset on value of type null on Digital Ocean

I have a project on Laravel-5.8

public function store(StoreAppraisalGoalRequest $request)
{
 $appraisalStartDate = Carbon::parse($request->appraisal_start_date);
 $appraisalEndDate = Carbon::parse($request->appraisal_end_date);        
 $userCompany = Auth::user()->company_id;
 $employeeId = Auth::user()->employee_id;
  $identities = DB::table('appraisal_identity')->select('id','appraisal_name')->where('company_id', $userCompany)->where('is_current', 1)->first();
  $employees = DB::table('hr_employees')->select('id')->where('id', $employeeId)->first();
   try {
     $goal = new AppraisalGoal();
     $goal->goal_type_id             = $request->goal_type_id;
     $goal->appraisal_identity_id    = $request->appraisal_identity_id;
     $goal->employee_id              = $request->employee_id;  //$employeeId;   //$request->employees_id
     $goal->weighted_score           = $request->weighted_score;
     $goal->goal_title               = $request->goal_title;
     $goal->goal_description         = $request->goal_description;
    
         if ($request->appraisal_doc != "") {
             $appraisal_doc = $request->file('appraisal_doc');
             $new_name = rand() . '.' . $appraisal_doc->getClientOriginalExtension();
             $appraisal_doc->move(public_path('storage/documents/appraisal_goal'), $new_name);
            // $arr['appraisal_doc'] = $new_name;
             $goal->appraisal_doc = $new_name;
        }   
    $goal->save();        
    
    foreach ( $request->activity as $key => $activity){
        $startDate = Carbon::parse($request->start_date[$key]);
        $endDate = Carbon::parse($request->end_date[$key]);

        $goaldetail = new AppraisalGoalDetail();

        $goaldetail->kpi_description            = $request->kpi_description[$key];
        $goaldetail->appraisal_doc              = $request->application_doc[$key];
        $goaldetail->activity                   = $request->activity[$key];  
        $goaldetail->start_date                 = $startDate ->toDateTimeString();
        $goaldetail->end_date                   = $endDate->toDateTimeString();                  
        $goaldetail->appraisal_goal_id          = $goal->id;
        $goaldetail->appraisal_identity_id      = $goal->appraisal_identity_id;
        $goaldetail->employee_id                = $goal->employee_id;
        $goaldetail->save();
     }
    $min_date = AppraisalGoalDetail::select('start_date')->where('appraisal_goal_id', $goal->id)->min('start_date');
    $max_date = AppraisalGoalDetail::select('end_date')->where('appraisal_goal_id', $goal->id)->max('end_date');   
    $parentid = AppraisalGoalType::select('parent_id')->whereNotNull('parent_id')->where('company_id', $userCompany)->where('id', $goal->goal_type_id)->first();
     
     $goal->update([
         'appraisal_start_date' => $min_date,
         'appraisal_end_date'   => $max_date,
         'parent_id'            => $parentid->parent_id
             ]);

     
        Session::flash('success', 'Appraisal Goal is created successfully');
        return redirect()->route('appraisal.appraisal_goals.index');
  } catch (Exception $exception) {
         Session::flash('danger', 'Appraisal Goal creation failed!');
        return redirect()->route('appraisal.appraisal_goals.index');
  }
}

When I deployed to Digital Ocean, I found out that only

    $goal = new AppraisalGoal();
    $goal->goal_type_id             = $request->goal_type_id;
    $goal->appraisal_identity_id    = $request->appraisal_identity_id;
    $goal->employee_id              = $request->employee_id;  //$employeeId;   //$request->employees_id
    $goal->weighted_score           = $request->weighted_score;
    $goal->goal_title               = $request->goal_title;
    $goal->goal_description         = $request->goal_description;
    
         if ($request->appraisal_doc != "") {
             $appraisal_doc = $request->file('appraisal_doc');
             $new_name = rand() . '.' . $appraisal_doc->getClientOriginalExtension();
             $appraisal_doc->move(public_path('storage/documents/appraisal_goal'), $new_name);
            // $arr['appraisal_doc'] = $new_name;
             $goal->appraisal_doc = $new_name;
        }   
   $goal->save();     

was save into the database. But,

        $goaldetail = new AppraisalGoalDetail();

        $goaldetail->kpi_description            = $request->kpi_description[$key];
        $goaldetail->appraisal_doc              = $request->application_doc[$key];
        $goaldetail->activity                   = $request->activity[$key];  
        $goaldetail->start_date                 = $startDate ->toDateTimeString();
        $goaldetail->end_date                   = $endDate->toDateTimeString();                  
        $goaldetail->appraisal_goal_id          = $goal->id;
        $goaldetail->appraisal_identity_id      = $goal->appraisal_identity_id;
        $goaldetail->employee_id                = $goal->employee_id;
        $goaldetail->save();
     }

was not saved. But on my local system everything works fine.

However, when I debugged using:

dd($exception->getMessage());

and set debug on server to true, I got this error:

"Trying to access array offset on value of type null"

How do I resolve it.

Thank you