fix: GraphQL playground shows blank page (#9858)

Author: cryingraven

src/GraphQL/ParseGraphQLServer.js

@@ -167,14 +167,14 @@ class ParseGraphQLServer {
            new window.EmbeddedSandbox({
              target: "#sandbox",
              endpointIsEditable: false,
-             initialEndpoint: "${JSON.stringify(this.config.graphQLPath)}",
+             initialEndpoint: ${JSON.stringify(this.config.graphQLPath)},
              handleRequest: (endpointUrl, options) => {
               return fetch(endpointUrl, {
                 ...options,
                 headers: {
                     ...options.headers,
-                    'X-Parse-Application-Id': "${JSON.stringify(this.parseServer.config.appId)}",
-                    'X-Parse-Master-Key': "${JSON.stringify(this.parseServer.config.masterKey)}",
+                    'X-Parse-Application-Id': ${JSON.stringify(this.parseServer.config.appId)},
+                    'X-Parse-Master-Key': ${JSON.stringify(this.parseServer.config.masterKey)},
                 },
               })
             },