fit property Null safety
How a flexible child is inscribed into the available space.
If flex is non-zero, the fit determines whether the child fills the space the parent makes available during layout. If the fit is FlexFit.tight, the child is required to fill the available space. If the fit is FlexFit.loose, the child can be at most as large as the available space (but is allowed to be smaller).
final FlexFit fit;