_getDatasourceByName null check

src/module.ts

@@ -614,6 +614,9 @@ class GraphCtrl extends MetricsPanelCtrl {
   }
 
   private async _getDatasourceByName(name: string) {
+    if(name === null) {
+      throw new Error('Trying to get datasource with NULL name');
+    }
     if(this._datasources[name] === undefined) {
       const datasource = await this.backendSrv.get(`/api/datasources/name/${name}`);
       return datasource;