fix(default datasource): minor fix for handling of default datasource

pkg/api/frontendsettings.go

@@ -98,7 +98,7 @@ func getFrontendSettingsMap(c *middleware.Context) (map[string]interface{}, erro
 	}
 
 	if defaultDatasource == "" {
-		defaultDatasource = "grafana"
+		defaultDatasource = "-- Grafana --"
 	}
 
 	jsonObj := map[string]interface{}{

public/app/panels/singlestat/module.js

@@ -37,6 +37,7 @@ function (angular, app, _, TimeSeries, kbn, PanelMeta) {
     // Set and populate defaults
     var _d = {
       links: [],
+      datasource: null,
       maxDataPoints: 100,
       interval: null,
       targets: [{}],