portraitLayout property Null safety
override
Implementation
@override
List<List<CalculatorButton?>> get portraitLayout => [
[sqrt, eX, tenX, yX, reciprocal, onOff],
[sst, gto, sin, cos, tan, chs],
[rs, gsb, rdown, xy, bsp, eex],
[sto, rcl, n7, n8, n9, div],
[fShift, gShift, n4, n5, n6, mult],
[null, null, n1, n2, n3, minus],
[null, null, n0, dot, sum, plus],
];