使用代码优先在asp.net中列出网页中的IdentityRoles时出现NullReferenceException

var roleManager = new RoleManager<IdentityRole>(new RoleStore<IdentityRole>(new ApplicationDbContext()));

var roleManager = new RoleManager<IdentityRole>(new RoleStore<IdentityRole>(new ApplicationDbContext()));
roleManager.Create(new IdentityRole("RoleName"));

<configuration>
  <appSettings>
    <add key="webpages:Version" value="3.0.0.0" />
    <add key="webpages:Enabled" value="false" />
    <add key="PreserveLoginUrl" value="true" />
    <add key="ClientValidationEnabled" value="true" />
    <add key="UnobtrusiveJavaScriptEnabled" value="true" />
  </appSettings>
  <system.web>
    <compilation debug="true" targetFramework="4.8" />
    <httpRuntime targetFramework="4.8" />
    <authentication mode="None" />
    <roleManager enabled="true" defaultProvider="AspNetSqlRoleProvider">
      <providers>
        <clear />
        <add name="AspNetSqlRoleProvider" type="System.Web.Security.SqlRoleProvider" connectionStringName="DefaultConnection" applicationName="/" />
      </providers>
    </roleManager>
  </system.web>
</configuration>