Adding a migration doesn't work

[vitusortner]

When trying to add a migration, an error is thrown, which is stating that The method 'add' was called on null.

This is caused by the fact, that the migrations field in the abstract TinanoDatabase class is instantiated with null but the generated code tries to add an element to the migrations list.

E/flutter ( 3741): [ERROR:flutter/shell/common/shell.cc(181)] Dart Error: Unhandled exception:
E/flutter ( 3741): NoSuchMethodError: The method 'add' was called on null.
E/flutter ( 3741): Receiver: null
E/flutter ( 3741): Tried calling: add(Instance of 'SchemaMigrationWithVersion')
E/flutter ( 3741): #0      Object.noSuchMethod (dart:core/runtime/libobject_patch.dart:50:5)
E/flutter ( 3741): #1      _$openMyDatabase (file:///Users/vitusortner/Development/Flutter/tinano/example/lib/database.g.dart:13:8)
E/flutter ( 3741): <asynchronous suspension>
E/flutter ( 3741): #2      MyDatabase.open (package:tinano_example/database.dart:10:39)
E/flutter ( 3741): #3      _MyHomePageState.initState.<anonymous closure> (file:///Users/vitusortner/Development/Flutter/tinano/example/lib/main.dart:56:36)
E/flutter ( 3741): <asynchronous suspension>
E/flutter ( 3741): #4      _MyHomePageState.initState (file:///Users/vitusortner/Development/Flutter/tinano/example/lib/main.dart:58:6)
E/flutter ( 3741): #5      StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:3787:58)
E/flutter ( 3741): #6      ComponentElement.mount (package:flutter/src/widgets/framework.dart:3653:5)
E/flutter ( 3741): #7      Element.inflateWidget (package:flutter/src/widgets/framework.dart:2937:14)
E/flutter ( 3741): #8      Element.updateChild (package:flutter/src/widgets/framework.dart:2740:12)
E/flutter ( 3741): #9      SingleChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:4796:14)
E/flutter ( 3741): #10     Element.inflateWidget (package:flutter/src/widgets/framework.dart:2937:14)
E/flutter ( 3741): #11     Element.updateChild (package:flutter/src/widgets/framework.dart:2740:12)
E/flutter ( 3741): #12     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3689:16)
E/flutter ( 3741): #13     Element.rebuild (package:flutter/src/widgets/framework.dart:3531:5)
E/flutter ( 3741): #14     ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:3658:5)
E/flutter ( 3741): #15     ComponentElement.mount (package:flutter/src/widgets/framework.dart:3653:5)
E/flutter ( 3741): #16     Element.inflateWidget (package:flutter/src/widgets/framework.dart:2937:14)
E/flutter ( 3741): #17     Element.updateChild (package:flutter/src/widgets/framework.dart:2740:12)
E/flutter ( 3741): #18     SingleChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:4796:14)
E/flutter ( 3741): #19     Element.inflateWidget (package:flutter/src/widgets/framework.dart:2937:14)
E/flutter ( 3741): #20     Element.updateChild (package:flutter/src/widgets/framework.dart:2740:12)
E/flutter ( 3741): #21     SingleChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:4796:14)
E/flutter ( 3741): #22     Element.inflateWidget (package:flutter/src/widgets/framework.dart:2937:14)
E/flutter ( 3741): #23     Element.updateChild (package:flutter/src/widgets/framework.dart:2740:12)
E/flutter ( 3741): #24     SingleChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:4796:14)
E/flutter ( 3741): #25     Element.inflateWidget (package:flutter/src/widgets/framework.dart:2937:14)
E/flutter ( 3741): #26     Element.updateChild (package:flutter/src/widgets/framework.dart:2740:12)
E/flutter ( 3741): #27     SingleChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:4796:14)
E/flutter ( 3741): #28     Element.inflateWidget (package:flutter/src/widgets/framework.dart:2937:14)
E/flutter ( 3741): #29     Element.updateChild (package:flutter/src/widgets/framework.dart:2740:12)
E/flutter ( 3741): #30     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3689:16)
E/flutter ( 3741): #31     Element.rebuild (package:flutter/src/widgets/framework.dart:3531:5)
E/flutter ( 3741): #32     ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:3658:5)
E/flutter ( 3741): #33     StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:3805:11)
E/flutter ( 3741): #34     ComponentElement.mount (package:flutter/src/widgets/framework.dart:3653:5)
E/flutter ( 3741): #35     Element.inflateWidget (package:flutter/src/widgets/framework.dart:2937:14)
E/flutter ( 3741): #36     Element.updateChild (package:flutter/src/widgets/framework.dart:2740:12)
E/flutter ( 3741): #37     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3689:16)
E/flutter ( 3741): #38     Element.rebuild (package:flutter/src/widgets/framework.dart:3531:5)
E/flutter ( 3741): #39     ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:3658:5)
E/flutter ( 3741): #40     ComponentElement.mount (package:flutter/src/widgets/framework.dart:3653:5)
E/flutter ( 3741): #41     Element.inflateWidget (package:flutter/src/widgets/framework.dart:2937:14)
E/flutter ( 3741): #42     Element.updateChild (package:flutter/src/widgets/framework.dart:2740:12)
E/flutter ( 3741): #43     ComponentElement.performRebuild (pack