We started getting the following Lucene error yesterday.

It crashes the entire website with 500 error and only restarting works.

Just to mention there was no changes to the code in the last 2-3 weeks and it just started appearing out of the blue.

Exception information: 
    Exception type: NullReferenceException 
    Exception message: Object reference not set to an instance of an object.
   at Lucene.Net.Index.SegmentReader.Norm.DecRef()
   at Lucene.Net.Index.SegmentReader.DoClose()
   at Lucene.Net.Index.IndexReader.DecRef()
   at Lucene.Net.Index.DirectoryReader.DoClose()
   at Lucene.Net.Index.IndexReader.DecRef()
   at Lucene.Net.Index.IndexReader.Close()
   at Examine.LuceneEngine.OpenReaderTracker.CloseStaleReaders(Directory dir, TimeSpan ts) in C:\projects\examine-qvx04\src\Examine\LuceneEngine\OpenReaderTracker.cs:line 44
   at Examine.LuceneEngine.Providers.LuceneSearcher.ReaderReopener.MaybeReopen() in C:\projects\examine-qvx04\src\Examine\LuceneEngine\Providers\LuceneSearcher.cs:line 618
   at Examine.LuceneEngine.Providers.LuceneSearcher.ReaderReopener.ScheduleReopen() in C:\projects\examine-qvx04\src\Examine\LuceneEngine\Providers\LuceneSearcher.cs:line 471
   at Examine.LuceneEngine.Providers.LuceneSearcher.ValidateSearcher() in C:\projects\examine-qvx04\src\Examine\LuceneEngine\Providers\LuceneSearcher.cs:line 368
   at Examine.LuceneEngine.Providers.LuceneSearcher.GetSearchFields() in C:\projects\examine-qvx04\src\Examine\LuceneEngine\Providers\LuceneSearcher.cs:line 245
   at UmbracoExamine.UmbracoExamineSearcher.GetSearchFields()
   at Examine.LuceneEngine.Providers.BaseLuceneSearcher.CreateSearchCriteria(String type, BooleanOperation defaultOperation) in C:\projects\examine-qvx04\src\Examine\LuceneEngine\Providers\BaseLuceneSearcher.cs:line 117
   at UmbracoExamine.UmbracoExamineSearcher.CreateSearchCriteria(String type, BooleanOperation defaultOperation)
   at Umbraco.Web.PublishedCache.XmlPublishedCache.PublishedMediaCache.GetUmbracoMediaCacheValues(Int32 id)
   at Umbraco.Core.Cache.DictionaryCacheProviderBase.<>c__DisplayClass10_0.<GetSafeLazy>b__0()



Request information: 
    Request URL: https://www.mydomain.com:443/umbraco/RenderMvc?id=25821 
    Request path: /umbraco/RenderMvc 
    User host address: ...
    User: ...
    Is authenticated: True 
    Authentication Type: ApplicationCookie 

Thread information: 
    Thread ID: 77 
    Thread account name: ...
    Is impersonating: False 
    Stack trace:    at Lucene.Net.Index.SegmentReader.Norm.DecRef()
   at Lucene.Net.Index.SegmentReader.DoClose()
   at Lucene.Net.Index.IndexReader.DecRef()
   at Lucene.Net.Index.DirectoryReader.DoClose()
   at Lucene.Net.Index.IndexReader.DecRef()
   at Lucene.Net.Index.IndexReader.Close()
   at Examine.LuceneEngine.OpenReaderTracker.CloseStaleReaders(Directory dir, TimeSpan ts) in C:\projects\examine-qvx04\src\Examine\LuceneEngine\OpenReaderTracker.cs:line 44
   at Examine.LuceneEngine.Providers.LuceneSearcher.ReaderReopener.MaybeReopen() in C:\projects\examine-qvx04\src\Examine\LuceneEngine\Providers\LuceneSearcher.cs:line 618
   at Examine.LuceneEngine.Providers.LuceneSearcher.ReaderReopener.ScheduleReopen() in C:\projects\examine-qvx04\src\Examine\LuceneEngine\Providers\LuceneSearcher.cs:line 471
   at Examine.LuceneEngine.Providers.LuceneSearcher.ValidateSearcher() in C:\projects\examine-qvx04\src\Examine\LuceneEngine\Providers\LuceneSearcher.cs:line 368
   at Examine.LuceneEngine.Providers.LuceneSearcher.GetSearchFields() in C:\projects\examine-qvx04\src\Examine\LuceneEngine\Providers\LuceneSearcher.cs:line 245
   at UmbracoExamine.UmbracoExamineSearcher.GetSearchFields()
   at Examine.LuceneEngine.Providers.BaseLuceneSearcher.CreateSearchCriteria(String type, BooleanOperation defaultOperation) in C:\projects\examine-qvx04\src\Examine\LuceneEngine\Providers\BaseLuceneSearcher.cs:line 117
   at UmbracoExamine.UmbracoExamineSearcher.CreateSearchCriteria(String type, BooleanOperation defaultOperation)
   at Umbraco.Web.PublishedCache.XmlPublishedCache.PublishedMediaCache.GetUmbracoMediaCacheValues(Int32 id)
   at Umbraco.Core.Cache.DictionaryCacheProviderBase.<>c__DisplayClass10_0.<GetSafeLazy>b__0()

It appears to happen straight after such error where for some reason it has problem getting the media root:

at Lucene.Net.Store.BufferedIndexInput.ReadBytes(Byte[] b, Int32 offset, Int32 len, Boolean useBuffer) 
at Lucene.Net.Index.SegmentReader.Norm.Bytes() at Lucene.Net.Index.SegmentReader.GetNorms(String field) 
at Lucene.Net.Index.SegmentReader.Norms(String field) at Lucene.Net.Search.TermQuery.TermWeight.Scorer(IndexReader reader, Boolean scoreDocsInOrder, Boolean topScorer) at Lucene.Net.Search.BooleanQuery.BooleanWeight.Scorer(IndexReader reader, Boolean scoreDocsInOrder, Boolean topScorer) at Lucene.Net.Search.BooleanQuery.BooleanWeight.Scorer(IndexReader reader, Boolean scoreDocsInOrder, Boolean topScorer) at Lucene.Net.Search.IndexSearcher.Search(Weight weight, Filter filter, Collector collector) 
at Examine.LuceneEngine.SearchResults.DoSearch(Query query, IEnumerable`1 sortField, Int32 maxResults) in C:\projects\examine-qvx04\src\Examine\LuceneEngine\SearchResults.cs:line 110 
at Examine.LuceneEngine.Providers.BaseLuceneSearcher.Search(ISearchCriteria searchParams, Int32 maxResults) in C:\projects\examine-qvx04\src\Examine\LuceneEngine\Providers\BaseLuceneSearcher.cs:line 182 
at Umbraco.Web.PublishedCache.XmlPublishedCache.PublishedMediaCache.GetAtRoot(UmbracoContext umbracoContext, Boolean preview) 
at Umbraco.Web.PublishedCache.ContextualPublishedCache`1.GetAtRoot(Boolean preview) 
at Umbraco.Web.PublishedContentQuery.TypedMediaAtRoot() 
at MyDomain.Services.MediaService.GetRootMedia(String mediaFolder) 
at MyDomain.Services.MediaService.GetSupplierMediaFolderId(IPublishedContent supplierContent) 
at MyDomain.Controllers.MemberController.<ProductImageSave>d__25.MoveNext()

The only thing that this bit of code does is:

    public IPublishedContent GetRootMedia(string mediaFolder)
    {
        var mediaRoots = _umbHelper.TypedMediaAtRoot();
        return mediaRoots.FirstOrDefault(f => f.Name == mediaFolder);
    }

mediaRoots returns empty collection despite there are a few media folders at root. This seems to happen every some time though - not every time or not for every member.

var mediaRoots = _umbHelper.TypedMediaAtRoot();

returns empty list asa well.

Umbraco 7.15.3