Attempt to de reference a null object error - salesforce

Question

On line acct.AmountX__c += amtX this and acct.AmountY__c += amtY this it is giving me an error. The trigger is on after insert and after update. When i am inserting any record in contact then it is giving me an error i don't know why. Why should 'acct' be initialized if i am not wrong this error comes when you have not initialized something.
public class ContactAccountRollUpAmount {
public static void onAfterInsertOrUpdate(List<Contact> cont){
contAccRollUpAmount(cont);
}
public static void contAccRollUpAmount(List<Contact> contactList){
List<Contact> cont = new List<Contact>([Select AccountId, Amount_Type__c, Amount__c
from Contact
where id in: contactList]);
List<Account> accList = new List<Account>([Select id, AmountX__c, AmountY__c
from Account Limit 50000]);
List<Account> accListToUpdate = new List<Account>();
for(Account acct: accList){
Double amtX = 0;
Double amtY = 0;
system.debug('#########'+acct);
for(Contact con: cont){
if(con.AccountId == acct.Id){
if(con.Amount__c != NULL){
if(con.Amount_Type__c == 'AmountX'){
amtX = amtX + con.Amount__c;
} else if(con.Amount_Type__c == 'AmountY'){
amtY = amtY + con.Amount__c;
}
}
}
}
if(amtX != NULL){
acct.AmountX__c += amtX;
}
if(amtY != NULL){
acct.AmountY__c += amtY;
}
accListToUpdate.add(acct);
}
if(accListToUpdate.size()>0){
update accListToUpdate;
}
}
}

Answer 1

The problem is not that the 'acct' is not initialized, its the field 'AmountX__c' and 'AmountY__c'.
As I can see you have queried max of 50000 records, out of these records some of them have 'Null' values for the above two fields.
I suggest please check whether the above two fields have 'Null' values for some records or not.