portraitLayout property Null safety
override
Implementation
@override
List<List<CalculatorButton?>> get portraitLayout => [
[onOff, rdown, xy, bsp, fShift, gShift],
[gsb, gto, hex, dec, oct, bin],
[a, b, c, d, e, f],
[rs, sst, n7, n8, n9, div],
[sto, rcl, n4, n5, n6, mult],
[null, null, n1, n2, n3, minus],
[null, null, n0, dot, chs, plus],
];