Failed to execute datasource query System.NullReferenceException while setting Datasource on Rendering

Question

Has anyone seen this error before or know how to solve it?

    996 16:35:46 WARN  Failed to execute datasource query System.NullReferenceException: Object reference not set to an instance of an object.
   at Sitecore.ContentSearch.SitecoreItemCrawler.IsExcludedFromIndex(SitecoreIndexableItem indexable, Boolean checkLocation)
   at Sitecore.ContentSearch.SitecoreItemCrawler.GetContextIndexRanking(IIndexable indexable)
   at System.Linq.Enumerable.WhereSelectListIterator`2.MoveNext()
   at System.Linq.Enumerable.Min(IEnumerable`1 source)
   at Sitecore.ContentSearch.AbstractSearchIndex.Sitecore.ContentSearch.Pipelines.GetContextIndex.IContextIndexRankable.GetContextIndexRanking(IIndexable indexable)
   at Sitecore.ContentSearch.Pipelines.GetContextIndex.FetchIndex.<>c__DisplayClass6_0.<RankContextIndexes>b__0(ISearchIndex i)
   at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext()
   at System.Linq.Buffer`1..ctor(IEnumerable`1 source)
   at System.Linq.OrderedEnumerable`1.<GetEnumerator>d__1.MoveNext()
   at System.Linq.Buffer`1..ctor(IEnumerable`1 source)
   at System.Linq.Enumerable.ToArray[TSource](IEnumerable`1 source)
   at Sitecore.ContentSearch.Pipelines.GetContextIndex.FetchIndex.GetContextIndex(IIndexable indexable, GetContextIndexArgs args)
   at Sitecore.ContentSearch.Pipelines.GetContextIndex.FetchIndex.Process(GetContextIndexArgs args)
   at (Object , Object[] )
   at Sitecore.Pipelines.CorePipeline.Run(PipelineArgs args)
   at Sitecore.Pipelines.DefaultCorePipelineManager.Run(String pipelineName, PipelineArgs args, String pipelineDomain)
   at Sitecore.Abstractions.CorePipelineWrapper.Run(String pipelineName, PipelineArgs args)
   at Sitecore.ContentSearch.Pipelines.GetContextIndex.GetContextIndexPipeline.Run(ICorePipeline pipeline, GetContextIndexArgs args)
   at Sitecore.Mvc.ExperienceEditor.DatasourceValidator.DatasourceValidator.IsDatasourceValid(String dataSource, Database database)
 996 16:35:46 WARN  '{9F469FDF-5697-47F7-8D55-01FBCD64E534}' is not valid datasource for master or user does not have permissions to access.

I am seeing this error when I try to set a DataSource on a rendering. I have tried this both via the Experience Editor and by setting it in the Layout manager.

Answer 1

Ok this was a stupid mistake. I haven't realized that my Solr instance has stopped running. The exception is thrown because it can't connect to Solr.

Starting Solr again solved the issue.