titleSpacing property Null safety
The spacing around title content on the horizontal axis. This spacing is applied even if there is no leading content or actions. If you want title to take all the space available, set this value to 0.0.
If this property is null, then AppBarTheme.titleSpacing of ThemeData.appBarTheme is used. If that is also null, then the default value is NavigationToolbar.kMiddleSpacing.
final double? titleSpacing;