Activity timer disappears after modifying a weekplan

[MathiasKRasmussen]

Describe the bug
When a timer(s) have been added to activities in a week plan, the timer(s) are deleted and sometimes the activity too when you do one of the following actions:

• The weekplan is edited (e,g, renamed)
• A new activity is added
• Reordering any activities
• Canceling a activity and then click "Fortryd"
• Copying an activity
• Deleting an activity
• Cancelling an activity from the weekplan screen

To Reproduce
Steps to reproduce the behavior:

1. Go to any weekplan
2. Click on an activity
3. Add a timer
4. Go back to the weekplan screen
5. Do one of the listed actions that creates the bug
6. See the timer(s) and the timer icon(s) disappear

Expected behavior
The activities with a timer before performing one of the actions should also have the timer afterwards

Actual behavior
The timer is not in the database after performing one of the actions

Additional context

• We know that the timers are not deleted when cancelling an activity from the activity screen
• All the actions that creates the bug calls the update method from the WeekApi class in the api-client (can be found in week_api.dart) These actions are also the only ones who call this method
• The update method sends a json string and receives a new json string
  • The json string that is send has the timer information, but the json string received has the timer(s) as null

[Lortemanden]

Solving this issue might also solve issue #112, aau-giraf/weekplanner#352 and aau-giraf/weekplanner#444