Update LockService.cs

resolving issue of 
"ClassName": "System.Threading.SynchronizationLockException",
"Message": "The read lock is being released without being held.",

LiteDB/Engine/Services/LockService.cs

@@ -53,8 +53,16 @@ public void ExitTransaction()
         {
             // if current thread are in reserved mode, do not exit transaction (will be exit from ExitExclusive)
             if (_transaction.IsWriteLockHeld) return;
-
-            _transaction.ExitReadLock();
+
+            //This can be called when a lock has either been released by the slim or somewhere else therefore there is no lock to release from ExitReadLock()
+            if (_transaction.IsReadLockHeld)
+            {
+                try
+                {
+                    _transaction.ExitReadLock();
+                }
+                catch { }
+            }
         }
 
         /// <summary>
@@ -148,4 +156,4 @@ public void Dispose()
             }
         }
     }
-}
+}