Invalid memory address or nil pointer dereference verifying jwt expiration time

I'm working on a login with golang using jwt token, everything so far is god, it checks what time is left on the token, if there is no token it send a message
But I have 2 problems, if the token is invalid or the expiration time is out it shows this error:

And the line that carry the error is this
claims := token.Claims.(*jwtCustomClaims)

my jwtCustomClaims variable is this:

type jwtCustomClaims struct {
User     string `json:"email"`
Nombre   string `json:"nombre"`
Apellido string `json:"apellido"`
Edad     int    `json:"edad"`
Genero   string `json:"genero"`
Rol      string `json:"rol"`
jwt.StandardClaims
}

with this struct is also generated the token, and the full function to check the token is this

func ValidateToken(tokenString string, secretKey string, c *fiber.Ctx) (*jwt.Token, error) {
token, err := jwt.ParseWithClaims(tokenString, &jwtCustomClaims{}, func(token *jwt.Token) (interface{}, error) {
return []byte(secretKey), nil
})
if err != nil {
if err == jwt.ErrSignatureInvalid {
return nil, err
}
return nil, c.JSON(fiber.Map{
"message": "Unauthorized",
"code":    400,
})
}
_, ok := token.Claims.(*jwtCustomClaims)
if !ok || !token.Valid {
return nil, c.JSON(fiber.Map{
"message": "Unauthorized",
"code":    400,
})
}
return token, nil
}
func TokenMiddleware(c *fiber.Ctx) error {
tokenString := c.Get("Authorization")
if tokenString == "" {
return c.JSON(fiber.Map{
"message": "No token",
"code":    400,
})
}
// Separa el token del encabezado "Bearer"
tokenString = strings.ReplaceAll(tokenString, "Bearer ", "")
token, err := ValidateToken(tokenString, os.Getenv("SECRET"), c)
if err != nil {
return err
}
claims := token.Claims.(*jwtCustomClaims)
expiresIn := time.Until(time.Unix(claims.ExpiresAt, 0))
if expiresIn > 0 {
return c.JSON(fiber.Map{
"message":    "Token is valid",
"expires_in": expiresIn.String(),
"is_expired": false,
})
//return c.Next()
} else {
return c.JSON(fiber.Map{
"message":    "Token is valid but expired",
"expires_in": 0,
"is_expired": true,
})
}
}

To check if it validate the token i just try to put another type of token, from an other proyect and write it wrong just to test but it send a console error. And for the expiration time i look the error on the web and a solution to this is to write it like this if claims == nil but it didn't work, I don't know what else to do

Please test it:

func ValidateToken(tokenString string, secretKey string, c *fiber.Ctx) (jwtCustomClaims, error) {
var claims jwtCustomClaims
token, err := jwt.ParseWithClaims(tokenString, &claims, func(token *jwt.Token) (interface{}, error) {
return []byte(secretKey), nil
})
if err != nil {
if err == jwt.ErrSignatureInvalid {
return jwtCustomClaims{}, err
}
return jwtCustomClaims{}, c.JSON(fiber.Map{
"message": "Unauthorized",
"code":    400,
})
}
if !token.Valid {
return jwtCustomClaims{}, c.JSON(fiber.Map{
"message": "Unauthorized",
"code":    400,
})
}
return claims, nil
}
func TokenMiddleware(c *fiber.Ctx) error {
.
.
.
claims, err := ValidateToken(tokenString, os.Getenv("SECRET"), c)
if err != nil {
return err
}
expiresIn := time.Until(time.Unix(claims.ExpiresAt, 0))
.
.
.
}