I have a master object and it's detail.When this code executes, it is crashing at mo.Contact != null complaining about null pointer reference - i.e master object is null.

I fail to see why this would be, given that I got the master object through a query to database and put it in a map. I am also assuming that salesforce would never try to insert/update a detail object unless the master object field was non-null.

Additional info:

I  am UPSERTING the detail object using DATA LOADER. So I have an external id on detail object. I also have an external id on the parent object and I am matching the parent based on external id of the parent. When the parent object has been deleted, I usually get an external id not found error.

trigger updateDetailObject on Detail_Object__c (before insert, before update) {
     
    Detail_Object__c[] DetailObjects = Trigger.New;
    
    Set<Id> MasterObjectIds = new Set<Id> ();

    for (Detail_Object__c do:DetailObjects ) {
       MasterObjectIds.add(do.Master_Object__c);
    }

   Map<Id, Master_Object__c> MasterObjectMap = new Map<Id, Master_Object__c> ([select Contact__c from Master_Object__c where id in :MasterObjectIds]);
 
   for (Detail_Object__c do:DetailObjects) {    
       Master_Object__c mo = MasterObjectMap.get(do.Master_Object__c);
       if (mo.Contact__c != null) { //crashes here. why would master object map not have the entry in it? 
          ...
       }
   }
}