Null Observer Exception (#161)

* Null Observer Exception

This need further investigation

* DefaultRepositoryMonitor.cs cleaning

* add comment

src/RepoM.Api/Git/DefaultRepositoryMonitor.cs

@@ -200,6 +200,12 @@ public void Reset()
 
         foreach (IRepositoryObserver observer in _repositoryObservers.Values)
         {
+            // ReSharper disable once ConditionIsAlwaysTrueOrFalseAccordingToNullableAPIContract
+            if (observer == null)
+            {
+                _logger.LogError("Null Observer in _repositoryObservers detected");
+                continue;
+            }
             observer.Stop();
             observer.Dispose();
         }
@@ -285,7 +291,7 @@ private void OnRepositoryChangeDetected(IRepository repo)
             _logger.LogDebug("{Method} - create observer {Head}", nameof(OnRepositoryChangeDetected), repo.Path);
             CreateRepositoryObserver(repo, path);
 
-            // use that delay to prevent a lot of sequential writes 
+            // use that delay to prevent a lot of sequential writes
             // when a lot of repositories get found in a row
             _storeFlushTimer.Change(5000, Timeout.Infinite);
         }
@@ -349,4 +355,4 @@ private enum KnownRepositoryNotifications
         WhenFound = 1,
         WhenNotFound = 2,
     }
-}
+}