Found a number of postings to resolve this error in the ULS log… When you try to search for results in the search center and you experience an error ‘Interal Exception Error’
For me, the issue wasn’t any thing I found searching around in google blogs or forums, lots of suggestions such as activing search features and creating new search sites etc…, or IIS resets, reboots and I dont need to reprovision the Search service application!
Error
CoreResultsWebPart::OnInit: Exception initializing: System.NullReferenceException: Object reference not set to an instance of an object
Diagnostics
Had this issue twice with two different methods of remedies. I have found the simple fix by finding the application pool account that is hosting the Search admin query component to be in the stopped state in IIS server and when its started it fails again after a new search query.
Resolution
1. Simply changing/generating a new password in SharePoint managed accounts section for the search service account running under the admin or query application pool account (or the account that is stopped in IIS) and then performing an IISReset.
2. Another resolution is to create new application pools in SharePoint for the search service and its up to you if you want to specify new identity accounts or use the same ones. Once the new application pools are created for the search service application you can verify in IIS that they are running in the started state and while you are in IIS, check the search service guids exists under the SharePoint Web Services virtual directory and the SearchService and web.config file are created.
Once this is done, IISRESET or Reboot.
SharePoint 
you are really great.it worked thank you so much