Fail Importing expo font

I'm getting this error when trying to import a font with expo font. null is not an object" (evaluating 'dispatcher.useState') But the error is for sure related to my font import.

import { NavigationContainer, useNavigation } from '@react-navigation/native';
import { FontAwesome } from '@expo/vector-icons';
import { MaterialCommunityIcons } from '@expo/vector-icons';
import {createStackNavigator} from '@react-navigation/stack';
import { AuthContext } from "./context";
import BottomTabNavigator from "./navigation/tabsNav";
import * as firebase from "firebase";
import Myapp from './instanceFirebase';
import AuthStackScreen from './navigation/authStack';
import { ActivityIndicator } from 'react-native'
import FlashMessage from "react-native-flash-message";


const instanceFirebase = Myapp;


import "firebase/functions";
import "firebase/firestore";
import * as Font from 'expo-font';
console.log("font", Font)


const [fontState, setFontState] = React.useState(false);



const MainStack = createStackNavigator();
const MainStackScreen = () => {
  return(
    
    <MainStack.Navigator initialRouteName="mainPage" >
        <MainStack.Screen 
          name="mainPage"
          component={BottomTabNavigator} />
    </MainStack.Navigator>   
  )
}

const RootStack = createStackNavigator();
const RootStackScreen = ({ userToken }) => (
  <RootStack.Navigator headerMode="none">
    {userToken ? (
      <RootStack.Screen
        name="App"
        component={MainStackScreen}
      />
    ) : (
      <RootStack.Screen
        headerMode="none"
        name="Authstack"
        component={AuthStackScreen}
        options={{
          animationEnabled: false
        }}
      />
    )}
  </RootStack.Navigator>
);




export default function token() {
  const [userToken, setUserToken] = React.useState(null);
  const Context = {
    instanceFirebase: instanceFirebase,
    signOut: () => {
      setUserToken(null);
    },
    signIn:() => {
      setUserToken(true);
    }
  }
  
  React.useEffect(() => {
    const fetchFont = async () => {
      await Font.loadAsync({
        Jost: require('./assets/fonts/Jost-Regular.ttf'),
        Jost_black: require('./assets/fonts/Jost-Black.ttf')
      })
      setFontState(true)
    }
    fetchFont()


    firebase.auth().onAuthStateChanged(user => {
      if(user){
        setUserToken(true)
      } else {
        setUserToken(null)
      }
    })

  }, [])
 
  if (!fontState) {
    return <ActivityIndicator />
  } else {
  return (
    <AuthContext.Provider value={instanceFirebase}>
        <NavigationContainer>
          <RootStackScreen userToken={userToken} />
            <FlashMessage position="top" style={{zIndex:99, flex:1, marginTop:30}}/>
        </NavigationContainer>
        </AuthContext.Provider>
  )
 }
}

I checked that expo-font is well import, and it is fine. I don't understand what is happening, when i comment the code related to the import of the font my app just work. So my error is for sure related to the font import. I do things correctly ? Thanks per advance for your help