Tukukkk

I have a bunch of location-listeners such as follows:

exports.recountTotalCaloriesOnUpdate = functions.database.ref('/mealsOf/{userId}/{day}/meals/{hour}/{mealId}')

    .onUpdate(

        async(change,context) => {

            let uid= context.params.userId;

            let day= context.params.day;

            /*

            let res= await dbroot.ref(`mealsOf/${uid}/${day}`).push('totalCalories').set(0).then( (e) => {

                return null ;

            });

            */

            const collectionRef = change.after.ref.parent.ref.parent;

            //const counterRef = dbroot.ref(`mealsOf/${uid}/${day}`).child('totalCalories');

            let finalSum = await collectionRef.ref.once('value').then((snap) => {

                let sum = 0;

                snap.forEach((child) => {

                    console.log(child.val().numCalories);

                    sum = sum + parseInt(child.val().numCalories);

                });

                return sum;

            });

            /*

            return await counterRef.ref.transaction((cnt) => {

                 return finalSum;

            });

            */

            return await dbroot.ref(`mealsOf/${uid}/${day}/totalCalories`).set(finalSum).then( (a) => {

                return null ;

            });

        }

    );

My DB-schema looks like follows:

For example, when the first meal even for a particular day is created, I do understand that totalCalories field for it may not yet exists, and more often than not I get the error referring to return await dbroot.ref(mealsOf/${uid}/${day}/totalCalories).set(finalSum).then( (a) => {
return null ;
});-line in the above code that says that everything before .set is NaN. I tried .push(<childneme>).set(0), too, but this error persists. I have many triggers that try to keep the DB in consistent state, e.g. cleaning up after functions.auth.user().onDelete() is called. So I think this .onWrite()-trigger might be interfering when a data being wiped out.
My question is, is the above code looks about correct for keeping the totalCalories field in consistent state?

EDIT: my guess is that the trigger is confused: when a user is deleted, my wipe-out function tries to delete the branch mealsOf/<userId> altogether, but this is considered as a write operation on all of its children, that is why the above trigger is launched and it sets the totalCalories to zero, but the reference is gone. That would explain why sometimes the mealsOf/<userId> branch would persist, whereas all the user-related data are gone after the deletion.