Flutter for loop containing .then statement completing first half then second half - flutter

Question

I have a list called wastedProducts which I want to iterate through and create a new list, wastedProductsSet based on each item in the original list.
This is the code I have to do that:
for (var productHeld in wastedProducts) {
if (wastedProductsSetNames.contains(productHeld.masterCat)) {
print("duplicate ${productHeld.masterCat}");
} else {
_wastedCount = wastedProducts
.where((p) => p.masterCat == productHeld.masterCat)
.fold(0,(amountWasted, product) => amountWasted + product.amountWasted);
_masterCat = Firestore.instance
.collection('masterCategories')
.document(productHeld.masterCat);
// repeats the above for each item before completing the below for each item
_masterCat.get().then(
(value) {
baseUnit = value.data['baseUnit'];
if (baseUnit == null) {
baseUnit = '';
} else {
baseUnit = value.data['baseUnit'];
}
wastedProductsSet.add(
WastedProduct(
productName: productHeld.masterCat,
wastedCount: _wastedCount.toInt(),
baseUnit: baseUnit,
),
);
wastedProductsSetNames.add(productHeld.masterCat);
},
);
}
}
Based on the print statements, I can see it is completing the code up to the _masterCat.get().then( line and doing that for each item in wastedProducts, then completing the code below _masterCat.get().then( for each item.
I assume it must have something to do with the asynchronous nature of the .then but cannot work out what the problem is.
I originally was using .forEach instead of for (var productHeld in wastedProducts) but changed based on the answer in this post My async call is returning before list is populated in forEach loop.