flutter – I receive an "invalid constant value" when I try to add a style theme to the title in BottomNavigationBarItem

I am adding a theme to the title in BottomNavigationBarItem and I receive the error "Invalid constant value".

I know that it is not the type of theme I am using because I was able to divide my theme method and use the individual constants in the following BottomNavigationBarItem. Hmmm, I was going to post an image that shows exactly what I want to say here, but I guess I still can't.

This is the code that tells me that I have invalid constant values

BottomNavigationBarItem(
            icon: Icon(CustomIcons.wrestlingring2),
            title: Text('Event', style: Theme.of(context).textTheme.button),
          ),

This code is where I added the constant values ​​directly to the style from my buttonTheme method on my style page

BottomNavigationBarItem(
            icon: Icon(CustomIcons.wrestling),
            title: Text('Match Details', style: TextStyle(fontFamily: montserratFont, fontSize: smallTextSize, color: textColorLight)),
          ),

These are the constant values ​​and the theme of the style page button

const smallTextSize = 14.0;
const Color buttonTextColour = Color(0xffdc0000);
const String montserratFont = 'Montserrat';

const buttonTextStyle = TextStyle(
  fontFamily: montserratFont,
  fontSize: buttonTextSize,
  color: textColorLight,
);

I noticed in the flutter code that BottomNavigationBarItem is a constant and I thought this could be the problem

const BottomNavigationBarItem({
    @required this.icon,
    this.title,
    Widget activeIcon,
    this.backgroundColor,
  }) : activeIcon = activeIcon ?? icon,
       assert(icon != null);

but then I noticed that so is RaisedButton, but it doesn't affect me to add a theme.

const RaisedButton({
    Key key,
    @required VoidCallback onPressed,
    ValueChanged onHighlightChanged,
    ButtonTextTheme textTheme,

If there is another way to use the theme with the title in NavigationBarItem, I would appreciate it if someone could point it out. I'd rather not have to fear the entire Buttom navigation bar.