lib/models/task.dart

@@ -27,7 +27,9 @@ class Task {
         reminders = (json['reminderDates'] as List<dynamic>)
             ?.map((r) => DateTime.parse(r))
             ?.toList(),
-        due = DateTime.parse(json['dueDate']),
+        due = json['dueDate'].toString() == 'null'
+            ? null
+            : DateTime.parse(json['dueDate']),
         description = json['description'],
         text = json['text'],
         done = json['done'],