My application compiles with no error and works with no problem but designer does not open any components. Any qlue?
Output window gives me this:
Starting Radzen.Server on port 4000
Connected to Radzen.Server
Waiting for any pending restore operations...
Waiting for packages to restore...
Restore complete.
Solution is up to date.
Opening project E:\RadzenApp\Bezra\Bezra.sln
Error during application startup: System.ArgumentNullException: Value cannot be null. (Parameter 'implementationInstance')
at System.ThrowHelper.Throw(String paramName)
at Microsoft.Extensions.DependencyInjection.ServiceCollectionServiceExtensions.AddSingleton[TService](IServiceCollection services, TService implementationInstance)
at RadzenProgram.Configure(IServiceCollection services, IConfiguration configuration)
at System.RuntimeMethodHandle.InvokeMethod(Object target, Void** arguments, Signature sig, Boolean isConstructor)
at System.Reflection.MethodBaseInvoker.InvokeDirectByRefWithFewArgs(Object obj, Span`1 copyOfArgs, BindingFlags invokeAttr)
Finished building E:\RadzenApp\Bezra\Bezra.csproj
Something in the application startup code (usually Program.cs) fails to run during design-time builds. As you know Radzen Blazor products eliminate certain code during design-time builds in order to avoid side effects such as database access, HTTP requests, file system IO. Image what would happen if your DB migration code or payments were running during design time 
Unfortunately we cannot recognize all such patterns and sometimes valid code gets removed which later leads to such exceptions. You can try using the preprocessor to hide certain parts from Program.cs until the build works again. Alternatively you can send us your Program.cs and we will investigate.
You can try using the preprocessor to hide certain parts from Program.cs until the build works again.
Good point. Thanks.
I did as you suggested but even my error disappears and the designer is still blank. What is the next step?
Are there any errors in the output pane? Does it work if you create a new Radzen Blazor Application? Does it work in Radzen Blazor Studio? I am not really sure what the problem could be with your app to be honest and it is close to impossible to tell without reproducing the issue somehow.
No error. Just:
Starting Radzen.Server on port 4001
Connected to Radzen.Server
Waiting for any pending restore operations...
Restore complete.
Solution is up to date.
Opening project E:\RadzenApp\Bezra\Bezra.sln
Finished building E:\RadzenApp\Bezra\Bezra.csproj
Yes it does work with the new Radzen Blazor Application. It does not work with Radzen Blazor Studio either, the same error applies. And yes indeed it is very hard to find an error when you could not reproduce it. Maybe some more logging in RBS can be of some help in a situation like this.
Getting no errors means that there is no exception being thrown anywhere in our implementation (otherwise it would have been logged).
I suspect your project has some unexpected nuance that RBS and RB4VS can't handle at the moment. The only way to tell more is to get more info about the app (kind of app, structure, projects etc) or get the source code to test it. We always delete customer's app source as soon as we finish troubleshooting so you shouldn't worry your code could leak. We can also sign an NDA if this would help you feel more comfortable. Alternatively you can try reproducing the issue in a stripped-down version of your app (without any sensitive data) and send us this version.
I solved my problem and RBS4VS and also RBS are working properly.
I do not know if this can help but I will explain my case and maybe this can be included in future improvements or just as known issues that have solutions.
In the beginning of Program.cs file I have declaration:
AppSettings appSettings = null;
and during further execution I initialized this class or not regards some conditions but never used it uninitialized. For some reasons this is not something that RBS can accept and it causes a blank design screen. When I change this declaration into
AppSettings appSettings = new AppSettings();
problem has vanished and RBS works as it should be. Although it causes some code modification for me, it is minor when it compares with using RBS in my everyday tasks.