Fix sandbox plugin initialisation

Summary: Fixes https://github.com/facebook/flipper/issues/1515

Reviewed By: passy

Differential Revision: D23572769

fbshipit-source-id: a541e2b4f626f520f39d21c3bf05d43ed919feb8

desktop/plugins/sandbox/index.tsx

@@ -73,7 +73,7 @@ export default class SandboxView extends FlipperPlugin<
 
   init() {
     this.client.call('getSandbox', {}).then((results: Array<Sandbox>) => {
-      this.dispatchAction({results, type: 'UpdateSandboxes'});
+      this.setState({sandboxes: results});
     });
   }