querySnapshot.value becomens null in _showCalendar()

[clockwiser]

querySnapshot.value becomes null and error appears eon emulator screen as below.

════════ Exception caught by widgets library ═══════════════════════════════════
The following NoSuchMethodError was thrown building LoadDataFromFireStore(dirty, state: LoadDataFromFireStoreState#86f10):
The getter 'keys' was called on null.
Receiver: null
Tried calling: keys

The relevant error-causing widget was
LoadDataFromFireStore
package:flutter_firebase_calendar/main.dart:19
When the exception was thrown, this was the stack
#0 Object.noSuchMethod (dart:core-patch/object_patch.dart:54:5)
#1 LoadDataFromFireStoreState._showCalendar
package:flutter_firebase_calendar/main.dart:92
#2 LoadDataFromFireStoreState.build
package:flutter_firebase_calendar/main.dart:83
#3 StatefulElement.build
package:flutter/…/widgets/framework.dart:4691
#4 ComponentElement.performRebuild
package:flutter/…/widgets/framework.dart:4574
...