appTheme top-level property Null safety

Implementation

NativeBaseTheme appTheme = NativeBaseTheme(
  fontSize: nbFontSizes,
  letterSpacing: baseLetterSpacings,
  fontWeight: baseNBFontWeights,
  lineHeight: baseLineHeights,
  component: baseComponents,
  spaces: nbBaseSpaces,
  sizes: nbBaseSizes,
  borders: nbBorders,
  colors: nbColor,
  shadows: nbShadow,
);