Flutter – function called using .then() is executed before the function on which then() is chained completes

In the onSelected function of a DropDownMenu widget, a function which returns a Future is called. After that Future completes, a regular function must be executed. In order to do this, am using the then keyword, as follows:

Serialize.deSerialize().then((e) => updateStatusbar());

The signature of the deserialize() method is:

static Future<void> deSerialize() async {

Within this method, there are several asynchronous calls using the await keyword, similar to the following:

await storage.read(key:'myKey'); 

The problem is that the updateStatusbar() method is called before the deSerialize() method has completed. I cannot call deSerialize() using await, because the DropDownMenu is defined within the build() method of widget, and that cannot be made async.

Adding print statements to verify the order of execution shows the following:

flutter: inside deSerialize 1
flutter: inside deSerxialize 2
flutter: inside deSerialize 3
flutter: inside deSerialize 4
flutter: inside method called from deSerialize 1
flutter: end of deSerialize
flutter: in updateStatusbar 1
flutter: in updateStatusbar 2
flutter: inside method called from deSerialize 2
flutter: inside method called from deSerialize 3
flutter: inside method called from deSerialize 4
flutter: inside method called from deSerialize 5
flutter: inside method called from deSerialize 6
flutter: inside method called from deSerialize 7
flutter: inside method called from deSerialize 8
flutter: inside method called from deSerialize 10
flutter: end of methods called from deSerialize

Most of the functions called using await within deSerialize() are called after the ones in updateStatusBar().

What is the correct way to chain these function calls so that deSerialize() only returns to then() after all the functions called within deSerialize() using await have finished?

Thanks very much