Access the Sitecore GraphQL Query UI Editor

 Recently, I was trying to dabble with graphql endpoints and when I tried to access, the endpoint - /sitecore/api/graph/items/master/ui, I got this page not found error:

I had set the web.config debug flag to true, which is one of the pre-requisites to access the graphql endpoint.

Since I wanted to access master db contents, I then enabled the Sitecore.Services.GraphQL.Content.Master.config under \App_Config\Sitecore\Services.GraphQL

Accessed the endpoint again:

This time some improvement but got the "Authentication required." message:

Although I was logged in to Sitecore, I still was getting the same error. Then, I decided to check the root cause as to when this error is thrown: 

Actual file:

This is an attribute added to GraphQL controller action methods:

Since this was my local environment, took the privilege to set the authentication flag to false in the Sitecore.Services.GraphQL.config:

Now, access the graph ql endpoint, some light in the dark: 

Results from a GraphQL query on the master db:

Respective config for the db must be enabled to access the corresponding GraphQL editor: