Error in c# code .Please check.

Question

public async Task<IActionResult> Register([FromBody]SignUp signUp)
{
try
{
SmartWatchUser user = await userManager.FindByEmailAsync(signUp.Email);
if (user != null)
return BadRequest("Username \"" + user.Email + "\" is already taken");
SmartWatchUser smartWatchUser = new SmartWatchUser
{
Email = signUp.Email,
FirstName = signUp.UserName?.Split(" ")?[0],
LastName = signUp.UserName?.Split(" ")?[1],
UserName = signUp.Email,
Status = UserStatus.New,
LockoutEnabled = false
};
var result = await userManager.CreateAsync(smartWatchUser, signUp.Password);
if (result.Succeeded)
{
var roleResult = await userManager.AddToRoleAsync(smartWatchUser, "AppUser");
return Ok("User Registered Successfully!");
}
else
return Ok(string.Join(",", result.Errors.Select(a=>a.Description).ToArray()));
}
catch (Exception ex)
{
//TODO logging
return StatusCode(500, ex.Message);
}
}

This is my signup code in .net core with entityfram work code first approach. I have face this error 'Index was outside the bounds of the array'. Please solve this what's problem this code.