toJson method Null safety

Implementation

@override
Map<String, dynamic> toJson() {
  return {
    "p": p,
    "pt": pt,
    "pr": pr,
    "pb": pb,
    "pl": pl,
    "px": px,
    "py": py,
    "m": m,
    "mt": mt,
    "mr": mr,
    "mb": mb,
    "ml": ml,
    "mx": mx,
    "my": my
  };
}