Sitecore Horizon - routing problems?

Question

I have installed Horizon. The window opens, but hangs (The loading gif in the center never goes away... site tree doesnt load...). I see in the logs:

Error in GraphQL query (parsing/system): GraphQL.ExecutionError: {"Message":
"No HTTP resource was found that matches the request URI 'https://myhost.com/sitecore/api/ssc/horizon/sites/-/getsites?name=mysite&sc_horizon=api'.",
"MessageDetail":"No type was found that matches the controller named 'sites'."}
2019-12-14T09:34:15.2313517-05:00 [ERR] (SerilogSample/PHLWLTR90V1MYD) Error processing GraphQL query
GraphQL.ExecutionError: Error trying to resolve name. ---> System.NullReferenceException: Object reference not set to an instance of an object.
at Sitecore.Horizon.API.Gateway.Schema.GraphTypes.SiteGraphType.<>c.<.ctor>b__1_0(ResolveFieldContext`1 context)
at GraphQL.Resolvers.FuncFieldResolver`2.GraphQL.Resolvers.IFieldResolver.Resolve(ResolveFieldContext context)
at GraphQL.Execution.ExecutionStrategy.ExecuteNodeAsync(ExecutionContext context, ExecutionNode node)
--- End of inner exception stack trace ---
2019-12-14T09:34:15.2317627-05:00 [ERR] (SerilogSample/PHLWLTR90V1MYD) Error processing GraphQL query
GraphQL.ExecutionError: Error trying to resolve language. ---> 
System.NullReferenceException: Object reference not set to an instance of an object.
at Sitecore.Horizon.API.Gateway.Schema.GraphTypes.SiteGraphType.<>c.<.ctor>b__1_3(ResolveFieldContext`1 context)
at GraphQL.Resolvers.FuncFieldResolver`2.GraphQL.Resolvers.IFieldResolver.Resolve(ResolveFieldContext context)
at GraphQL.Execution.ExecutionStrategy.ExecuteNodeAsync(ExecutionContext context, ExecutionNode node)
--- End of inner exception stack trace ---
2019-12-14T09:34:15.2320987-05:00 [ERR] (SerilogSample/PHLWLTR90V1MYD) Error processing GraphQL query
GraphQL.ExecutionError: Error trying to resolve startItem. ---> System.NullReferenceException: Object reference not set to an instance of an object.
at Sitecore.Horizon.API.Gateway.Schema.GraphTypes.SiteGraphType.ResolveStartItem(ResolveFieldContext`1 context)
at GraphQL.Execution.ExecutionStrategy.ExecuteNodeAsync(ExecutionContext context, ExecutionNode node)
--- End of inner exception stack trace ---

I have looked at ShowServicesConfig.aspx and I see that the Horizon SitesController is registered, so I am inclined to think its a routing problem? But I am unsure.

Update

To clarify:

I installed Sitecore 9.3. Sitecore, XConnect and Identity Server are all working. I installed Horizon to this instance of Sitecore. I had to mess with the parameters of the Horizon install script some, and still had a few problems. I ended up editing urls in the Horizon configs post install. For example, my Sitecore physical path and instance name do not match. The Horizon installer didnt like that at all.

Node and NPM and .Net Core hosting bundle are all installed.

Answer 1

The error here does not give a clue as to the issue. With the assistance of Sitecore Support I re-installed Horizon to no avail, but they seemed to think it had something to do with my Site Definitions, which it in fact did. It turns out my site definition was set for live mode, that is, pointing to the master database. As soon as I changed it to web, Horizon started working.

The way we found this was to create a new new dummy 'Home' page and disable my site definitions patch. Horizon came right up. So I diffed my Site Definitions with the example site definitions patch that comes with Sitecore and there it was.