Remove nullable and GitHub Actions warnings - #1

Possible null reference argument for parameter 'lastMove' in 'void BoardRenderer.Draw(Canvas canvas, int[,] board, bool pieceSelected, int? selectedRow, int? selectedColumn, IReadOnlyList<Move> highlightedLegalMoves, Move lastMove)'.

Converting null literal or possible null value to non-nullable type.

Because this call is not awaited, execution of the current method continues before the call is completed. Consider applying the 'await' operator to the result of the call.

Converting null literal or possible null value to non-nullable type.

Possible null reference argument for parameter 'text' in 'string BoardEditorController.BuildSafePositionFileName(string text)'.

Converting null literal or possible null value to non-nullable type.

Converting null literal or possible null value to non-nullable type.

Dereference of a possibly null reference.

Converting null literal or possible null value to non-nullable type.