Ship Station Connection Error

Hi!
I was trying to connect Ship Station, did everything by the guide, but got an error:

"Store Configuration
We received an error testing the Custom store: Invalid XML. Error: 'Root element is missing.' We received: ''

I have no idea which XML it means and how to fix it.
Please give me direction to think

Check that all your ShipStation settings are correct  - e.g.  plugin settings for Username and Password must match those in online ShipStation settings page.

When did you get the error - during checkout when it reached the shipping method selection page?

New York wrote:

Thank you for answer! I double checked - everything is correct.
I got the error when I click "Test connection" in ShipStation, here is the screenshoot: https://prnt.sc/12jmtst

Is ropapormayormayoreo.us reachable from the internet?  I can't see it.

New York wrote:

Yes, https://ropapormayoreo.us/ is reachable. You mention another URL

The put
https://ropapormayoreo.us/Plugins/ShipStation/Webhook
in browser address bar, and then look in the System Log for detailed error message.

New York wrote:

Log level: Error
Short message: Object reference not set to an instance of an object.
Full message:
System.NullReferenceException: Object reference not set to an instance of an object.
   at Nop.Plugin.Shipping.ShipStation.Controllers.ShipStationController.Webhook()
   at Microsoft.AspNetCore.Mvc.Infrastructure.ActionMethodExecutor.TaskOfIActionResultExecutor.Execute(IActionResultTypeMapper mapper, ObjectMethodExecutor executor, Object controller, Object[] arguments)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.<InvokeActionMethodAsync>g__Awaited|12_0(ControllerActionInvoker invoker, ValueTask`1 actionResultValueTask)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.<InvokeNextActionFilterAsync>g__Awaited|10_0(ControllerActionInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.Rethrow(ActionExecutedContextSealed context)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.<InvokeInnerFilterAsync>g__Awaited|13_0(ControllerActionInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeNextResourceFilter>g__Awaited|24_0(ResourceInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.Rethrow(ResourceExecutedContextSealed context)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeFilterPipelineAsync>g__Awaited|19_0(ResourceInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeAsync>g__Logged|17_1(ResourceInvoker invoker)
   at Microsoft.AspNetCore.Routing.EndpointMiddleware.<Invoke>g__AwaitRequestTask|6_0(Endpoint endpoint, Task requestTask, ILogger logger)
   at WebMarkupMin.AspNetCore5.WebMarkupMinMiddleware.ProcessAsync(HttpContext context, Boolean useMinification, Boolean useCompression)
   at WebMarkupMin.AspNetCore5.WebMarkupMinMiddleware.ProcessAsync(HttpContext context, Boolean useMinification, Boolean useCompression)
   at WebMarkupMin.AspNetCore5.WebMarkupMinMiddlewareBase.Invoke(HttpContext context)
   at StackExchange.Profiling.MiniProfilerMiddleware.Invoke(HttpContext context) in C:\projects\dotnet\src\MiniProfiler.AspNetCore\MiniProfilerMiddleware.cs:line 125
   at Microsoft.AspNetCore.Authorization.AuthorizationMiddleware.Invoke(HttpContext context)
   at Nop.Services.Authentication.AuthenticationMiddleware.InvokeAsync(HttpContext context)
   at Microsoft.AspNetCore.Localization.RequestLocalizationMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Session.SessionMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Session.SessionMiddleware.Invoke(HttpContext context)
   at Nop.Services.Installation.InstallUrlMiddleware.InvokeAsync(HttpContext context, IWebHelper webHelper)
   at Nop.Services.Common.KeepAliveMiddleware.InvokeAsync(HttpContext context, IWebHelper webHelper)
   at Microsoft.AspNetCore.ResponseCompression.ResponseCompressionMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Diagnostics.StatusCodePagesMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Diagnostics.StatusCodePagesMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Diagnostics.ExceptionHandlerMiddleware.<Invoke>g__Awaited|6_0(ExceptionHandlerMiddleware middleware, HttpContext context, Task task)

You are using default admin customer in shipstation or you have created from admin side ?

forefront wrote:

I used both, according to the instructions: https://prnt.sc/12msrs0
Or, maybe I did not understand your question right?

New York wrote:

Yes, what can we do more?
I sent detailed message and also delete the plugin and install it again,   but it did not help.