Hello,

High-level problem:

I'm trying to build a caculated filed that returns percent weight loss for patient accounts at 6 weeks mark. I did that through building a trigger and custom filed on the account object. Trigger works for some accounts and throw this error message (below) for other accounts.  

Error mesage return upon saving records:

I keep getting this error message upon saving editing/inserting new appointments (Child/Detail records) for persons account. 

Review all error messages below to correct your data.
Apex trigger ReturnPercentWeightLossAt6WeeksMark caused an unexpected exception, contact your administrator: ReturnPercentWeightLossAt6WeeksMark: execution of AfterInsert caused by: System.NullPointerException: Attempt to de-reference a null object: Trigger.ReturnPercentWeightLossAt6WeeksMark: line 42, column 1

(Please refer to the end of the page if you want a get a clearer idea of what I'm trying to accomplish by building this trigger)
 

Trigger ReturnPercentWeightLossAt6WeeksMark on Appointments__c(after insert, after update) {
    Set<Id> accountIds = new Set<Id>();

    for (Appointments__c appt : trigger.new) {
        accountIds.add(appt.Patient__c);
    }

    //Elimnate the the accounts that don't have IDs for
    accountIds.remove(null);

    //SOQL query that returns weight value at six weeks mark (Between 35 & 56; closer to 35) 
    if (!accountIds.isEmpty()) {
        List<Account> accountsToUpdate = new List<Account>();
        for (Account account : [
            Select Id,Initial_Weight__c,
                (
                    Select Scheduled_time__c, weight__c
                    From Dr_Chrono_appointments__r
                    WHERE Weight__c != NULL AND Status__c NOT IN ('Canceled', 'Cancelled') AND Days_since_first_appointment__c > 37 AND Days_since_first_appointment__c < 55  
                    Order by Scheduled_time__c asc
                    Limit 1
                )
            From Account
            Where Id in :accountIds
        ]) {
           
            //Assigning account's inital weight value to variable
            Decimal InitialWeight = NULL;    
            
            //Declare a decimal variable to store latest weight value 
            Decimal SixWeeksWeight = NULL;
            
            // Get(0) to return the first element in the list value
            if (!account.Dr_Chrono_appointments__r.isEmpty()) {
                SixWeeksWeight = account.Dr_Chrono_appointments__r.get(0).weight__c;
            }
            
            if (account.Initial_Weight__c != NULL) {
                InitialWeight = account.initial_weight__c;
            }
                
            Decimal WeightLostAtSixWeeks = InitialWeight - SixWeeksWeight;
            Decimal PercentWeightLoss = (WeightLostAtSixWeeks/InitialWeight)*100; 
            
            //if (SixWeeksWeight == NULL) {SixWeeksWeight = 0;
            //   } else {
            
            AccountsToUpdate.add(new Account(
                Id = account.Id,
                Percent_weight_loss_at_6_weeks__c = PercentWeightLoss
            ));
                    
            //}
                         
        }   
        
        update AccountsToUpdate;
        
        }
      }

Here is some background information on what I'm trying to do with this code:

I have two objects; Patients (persons accounts, Master object) and Appointments (Detail object) and I'm trying to build a trigger (With SOQL query) that returns percent weight loss at 6 weeks mark per account on a custom field (Percent_weight_loss_at_6_weeks__c) that is built on the account (Persons account) object:
 
Object #1: (Appointments)
 
Appointment ID  Account ID,   Account Name,  Apt Date,   Weight....etc.
-----------------------------------------------------------------------------------------------
Apts_001112       001                John                 01/01/2017     160
Apts_001114       002                Nicole               11/05/2016      180
Apts_001113       001                John                 04/05/2017     175
Apts_001115       003                Mark                 05/05/2017      190   
Apts_001116       002                Nicole               12/15/2016      200
...
...
...
 
Object #2: (Accounts)
 
 ID,      Name         Percent Weight Loss at 6 weeks. (This the metric that I'm trying to return in the account object) 
----------------------------------------------------------------------------------------------
 001     John          3%
 002     Nicole        2%
 003     Mark          3%     
----------------------------------------------------------------------------------------------
P.S:

The trigger worked for patients (Person accounts) who have existing appointments at 6 weeks mark (between 35 - 56) but didn't work; threw the above error message ^, for patients who don't have appointment at 6 weeks mark.