nbBaseSizes top-level property Null safety

Implementation

NBSizes nbBaseSizes = NBSizes(
  container: NBContainer(
    lg: 1024.0,
    sm: 640.0,
    md: 768.0,
    xl: 102.0,
  ),
  d3xs: 224.0,
  d2xs: 256.0,
  d2xl: 672.0,
  xs: 320.0,
  sm: 384.0,
  md: 448.0,
  lg: 512.0,
  xl: 576.0,
  spaces: nbBaseSpaces,
);