Hi I have the trigger below to populate a child field from a parent object field. Upon firing the trigger gives a NULL Pointer error. I have tried severally to for a solution to it but to no avail. Could someone help me with an explanation as to why the error and suggestion to a solution. Thanks.

trigger populateFieldOnCase on Case (before insert){

Set<id>parentIds = new Set<id>();

for(Case acase:trigger.new){

parentIds.add(acase.project__c);
}
Map<id,parent__c> pro = new Map<id,parentt__c>([select id,parentField__c from parent__c where id in :parentIds]);

for(case acase:trigger.new){

acase.childfield__c = pro.get(acase.id).parentField__c;

}
}