Actively checks if the lucene searcher has been disposed before creat…

…ing a search context, throws more informative error

src/Examine/LuceneEngine/Providers/LuceneSearcher.cs

@@ -89,6 +89,9 @@ public override Searcher GetLuceneSearcher()
 
         public override ISearchContext GetSearchContext()
         {
+            var searcher = GetLuceneSearcher();
+            if (searcher == null)
+                throw new InvalidOperationException($"Cannot create a {typeof(ISearchContext)}, the {Name} index either doesn't exist or the {typeof(LuceneSearcher)} has been disposed");
             return new SearchContext(FieldValueTypeCollection, GetLuceneSearcher());
         }
 
@@ -181,6 +184,7 @@ private bool ValidateSearcher()
         {
             //can't proceed if there's no index
             if (!IndexExistsImpl()) return false;
+            if (_disposed) return false;
 
             //TODO: Would be nicer if this used LazyInitializer instead of double check locking
 

src/Examine/LuceneEngine/Search/SearchContext.cs

@@ -10,8 +10,8 @@ public class SearchContext : ISearchContext
 
         public SearchContext(FieldValueTypeCollection fieldValueTypeCollection, Searcher searcher)
         {
-            _fieldValueTypeCollection = fieldValueTypeCollection;
-            Searcher = searcher;
+            _fieldValueTypeCollection = fieldValueTypeCollection ?? throw new System.ArgumentNullException(nameof(fieldValueTypeCollection));
+            Searcher = searcher ?? throw new System.ArgumentNullException(nameof(searcher));
         }
 
         public Searcher Searcher { get; }