Error during crossword generation

[Saggre]

According to the documentation

1. Go to packages/board_generator/tool
2. Create words.csv

word,"clue"
flutter,"Open source framework to build multi-platform applications"
firebase,"Mobile and web app development platform by Google"

3. Run dart pub get
4. Run dart asymmetrical_generation.dart

Output:

Sorting 1 words
Sorted 1 words
Unhandled exception:
Null check operator used on a null value
#0      SymmetricalCrosswordGenerator.seed (package:board_generator/src/generators/symmetrical_crossword_generator.dart:40:46)
#1      CrosswordGenerator.populate (package:board_generator/src/generators/crossword_generator.dart:32:5)
#2      main (file:///app/packages/board_generator/tool/symmetrical_generation.dart:38:31)
#3      _delayEntrypointInvocation.<anonymous closure> (dart:isolate-patch/isolate_patch.dart:297:19)
#4      _RawReceivePort._handleMessage (dart:isolate-patch/isolate_patch.dart:184:12)