Migrate to null safety

lib/globals.dart

lib/main.dart

lib/models/cesium_plus.dart

lib/models/change_pin.dart

lib/models/chest_data.dart

lib/models/chest_data.g.dart

lib/models/chest_provider.dart

lib/models/g1_wallets_list.dart

lib/models/g1_wallets_list.g.dart

lib/models/generate_wallets.dart

lib/models/home.dart

lib/models/my_wallets.dart

lib/models/search.dart

lib/models/wallet_data.dart

lib/models/wallet_data.g.dart

lib/models/wallet_options.dart

lib/models/wallets_profiles.dart

lib/screens/avatar_fullscreen.dart

lib/screens/common_elements.dart

lib/screens/history.dart