error The getter 'month' was called on null.

I have this problem when I try to refresh something from the table:

Exception has occurred.
NoSuchMethodError (NoSuchMethodError: The getter 'month' was called on null.
Receiver: null
Tried calling: month)

In my application I have a lot of users, and a calendar for each user (to put the day they enter).

i try using a single CalendarController () or make a fixed list to have a CalendarController for each one, but when I want to modify something I get the error that a month, I get null, for example, I have another calendar that would filter by days (to see which people entered that day). and the initial day works great, changing it I get this error. It also happens to me with the hot restart.

any clue? the calendar works great, i have the days of the firebase, and work fine. but filtering or refreshing have this problem.