Use 'late' for class fields that will be set by initState

[hawjo01]

Remove some unnecessary code because fields will not be null.

[github-actions]

LCOV of commit 466c424 during Analyze and Test on Pull Request #37

Summary coverage rate:
  lines......: 54.9% (246 of 448 lines)
  functions..: no data found
  branches...: no data found

Files changed coverage rate:
                                          |Lines       |Functions  |Branches    
  Filename                                |Rate     Num|Rate    Num|Rate     Num
  ==============================================================================
  lib/widgets/date_picker_input_field.dart| 0.0%     29|    -     0|    -      0
  lib/widgets/time_picker_input_field.dart| 0.0%     28|    -     0|    -      0