How to dismiss a dialog and a bottom sheet consecutively? - flutter

Question

I have a function that displays a bottom sheet. If one item of this bottom sheet is selected it is dismissed using Navigator.of(context).pop() and a dialog is shown. If the user taps on the "Cancel" button I want to dismiss the dialog, too, so I tried using Navigator.of(context).pop() again. But nothing happens and I get the error message Looking up a deactivated widget's ancestor is unsafe. I tried deleting the first one and now the dialog can be dismissed by pressing "Cancel" button but the bottom sheet is still there. Ideally was a way to make the app in the current form work but dismissing both at the same time would also be fine.
I would really appreciate any help.
Edit: Added code
List<Widget> exerciseSelectionItems(BuildContext context, String schoolClassId) {
List<Widget> items = new List<Widget>();
for (var i = 0; i < selectableExercises.length; i++) {
Exercise currentExercise = selectableExercises[i];
items.add(
ListTile(
leading: Icon(currentExercise.icon),
title: Text(currentExercise.name),
onTap: () {
Navigator.of(context).pop();
showDialog(
context: context,
child: AlertDialog(
title: Text(
AppLocalizations.of(context).translate('create_homework'),
),
content: Text(
AppLocalizations.of(context)
.translate('create_homework_message'),
),
actions: <Widget>[
FlatButton(
child: Text('OK'),
onPressed: () {
Database().addHomework(
schoolClassId,
Homework(i, DateTime.now().millisecondsSinceEpoch),
);
Navigator.of(context).pop();
},
),
FlatButton(
child: Text(
AppLocalizations.of(context).translate('cancel'),
),
onPressed: () {
Navigator.of(context).pop();
},
),
],
),
);
},
),
);
}
return items;}