Null Exception Issue When Working with Jwt Token

Hello Microsoft Team,

When i try to login by username and password. 

null exception error occur when controller hit opt.TokenValidationParameters = new TokenValidationParameters() which is available in startup.cs .

 opt.TokenValidationParameters = new TokenValidationParameters()
                {
                    ValidateIssuerSigningKey = true,
                    IssuerSigningKey = new SymmetricSecurityKey(Encoding.ASCII
                    .GetBytes(Configuration.GetSection("AppSettings:Token").Value)),
                    ValidateIssuer = false,
                    ValidateAudience = false
                };

Errors is

Exception has occurred: CLR/System.ArgumentNullException
An exception of type 'System.ArgumentNullException' occurred in System.Private.CoreLib.dll but was not handled in user code: 'String reference not set to an instance of a String.'
   at System.Text.Encoding.GetBytes(String s)
   at CI.API.Startup.<ConfigureServices>b__4_4(JwtBearerOptions opt) in D:\MyProject\CI.API\Startup.cs:line 67
   at Microsoft.Extensions.Options.ConfigureNamedOptions`1.Configure(String name, TOptions options)
   at Microsoft.Extensions.Options.OptionsFactory`1.Create(String name)
   at Microsoft.Extensions.Options.OptionsMonitor`1.<>c__DisplayClass11_0.<Get>b__0()
   at System.Lazy`1.ViaFactory(LazyThreadSafetyMode mode)

In startup.cs

 services.AddAuthentication(x =>
            {
                x.DefaultAuthenticateScheme = JwtBearerDefaults.AuthenticationScheme;
                x.DefaultChallengeScheme = JwtBearerDefaults.AuthenticationScheme;
            })
            .AddJwtBearer(opt =>
            {
                opt.TokenValidationParameters = new TokenValidationParameters()
                {
                    ValidateIssuerSigningKey = true,
                    IssuerSigningKey = new SymmetricSecurityKey(Encoding.ASCII
                    .GetBytes(Configuration.GetSection("AppSettings:Token").Value)),
                    ValidateIssuer = false,
                    ValidateAudience = false
                };
            });

In appsettings.json

"AppSettings":{"Token":"hey i am here"
  }

AuthController 

      [HttpPost("login")]
        public async Task<IActionResult> Login(LoginViewModel model)
        {
            var user = await _userManager.FindByNameAsync(model.UserName);
            var result = await _signInManager.CheckPasswordSignInAsync(user, model.Password, false);
            if (!result.Succeeded)
            {
                return BadRequest(result);
            }

            return Ok(new
            {
                result = result,
                token = await JwtTokenGenerator(user)
            });

        }




        public async Task<string> JwtTokenGenerator(User userInfo)
        {
            var claims = new List<Claim>
            {
            new Claim(ClaimTypes.NameIdentifier,userInfo.Id),
            new Claim(ClaimTypes.Name,userInfo.UserName)
          };

            var roles = await _userManager.GetRolesAsync(userInfo);
            foreach (var role in roles)
            {
                claims.Add(new Claim(ClaimTypes.Role, role));
            }

            var securityKey = new SymmetricSecurityKey(Encoding.ASCII
            .GetBytes(_config.GetSection("AppSettings:Token").Value));
            var credentials = new SigningCredentials(securityKey, SecurityAlgorithms.HmacSha512Signature);

            var tokenDescriptor = new SecurityTokenDescriptor
            {
                Subject = new ClaimsIdentity(claims),
                Expires = DateTime.Now.AddDays(1),
                SigningCredentials = credentials
            };

            var tokenHandler = new JwtSecurityTokenHandler();

            var token = tokenHandler.CreateToken(tokenDescriptor);
            return tokenHandler.WriteToken(token);


        }