bug: workout runner screen turns grey

[mschmidm]

Steps to reproduce:

• Install a fresh version of JAWT (delete data before).
• create a simple workout with for example one exercise
• start the workout
  --> workout runner screen turns grey

The following exception is thrown:

======== Exception caught by widgets library =======================================================
The following assertion was thrown building Builder:
Failed assertion: boolean expression must not be null

The relevant error-causing widget was: 
  MaterialApp file:///home/oxq/StudioProjects/just_another_workout_timer/lib/main.dart:21:41
When the exception was thrown, this was the stack: 
#0      _WorkoutPageState.buildTimetable.<anonymous closure>.<anonymous closure> (package:just_another_workout_timer/workout_runner.dart:184:54)
#1      ListMapView.forEach (dart:_internal/list.dart:242:8)
#2      _WorkoutPageState.buildTimetable.<anonymous closure> (package:just_another_workout_timer/workout_runner.dart:128:31)
#3      ListMapView.forEach (dart:_internal/list.dart:242:8)
#4      _WorkoutPageState.buildTimetable (package:just_another_workout_timer/workout_runner.dart:124:27)
...
====================================================================================================

The problem is that some SharedPreferences values (in this case 'halftime') are not set yet and are therefore null. The assertion in workout_runner.dart:184 then throws the exception.
This could be solved by adding a if null operator (??) but I think the cleanest solution is setting the default values in the main() method as done in my pull request.

Kind regards
oxq