Fix error bar displaying

Summary: The error was getting wiped accidentally if register device didn't have any error. In practice, meaning that the "flipper already running" message only flickered on the screen and then disappeared.

Reviewed By: passy

Differential Revision: D14576768

fbshipit-source-id: 1c7443f8f25ffa6a56f8d4994f3fef53bbb02c6d

src/reducers/connections.js

@@ -172,7 +172,7 @@ export default function reducer(
         // select device if none was selected before
         selectedDevice,
         ...selection,
-        error,
+        error: error || state.error,
       };
     }
     case 'UNREGISTER_DEVICES': {