Accessing Child Records from sObjects in Dynamic Queries Using Dynamic Apex

AccessingChildRecords.apex

String qstring ='Select id,Name,(select id,Lastname,Email from Contacts),(Select id,Name,Stagename from Opportunities) from Account where id=\'00190000004Awos\' Limit 1';
List<sobject> sobjectList = database.query(qstring);
for(sObject sb:sobjectList){
    System.debug('****Parent record-Account details:'+sb.get('Name'));
	//check if account has any contact child records
    List<sobject> contactChildObjects = sb.getSobjects('Contacts');
    System.debug('****Child Contact Records size:'+contactChildObjects.size());
    if(contactChildObjects.size()>0){
    	for(sobject con:contactChildObjects){
            system.debug('Contact Id:'+ con.get('Id'));
            system.debug('Contact Lastname:'+ con.get('Lastname'));
            system.debug('Contact Email:'+ con.get('Email'));
        }    
    }
	//check if account has any opportunities child records
    List<sobject> oppChildObjects = sb.getSobjects('Opportunities');
    System.debug('****opportunities Records size:'+oppChildObjects.size());
    if(oppChildObjects.size()>0){
    	for(sobject con:oppChildObjects){
            system.debug('opp Id:'+ con.get('Id'));
            system.debug('opp Name:'+ con.get('Name'));
            system.debug('opp Stage:'+ con.get('Stagename'));
        }    
    }
}

what to do if we want to fetch all account and related contact and opportunity records without id and limit.Iam getting null pointer exception while trying.