fix NullReferenceException when closing text editor immediately after committing.

src/Main/Base/Project/Src/Editor/IDocumentBaseVersionProvider.cs

@@ -67,6 +67,9 @@ namespace ICSharpCode.SharpDevelop.Editor
 		{
 			if (alreadyCalled) {
 				alreadyCalled = false;
+				// thread-safety: copy delegate reference into local variable
+				var actions = this.actions;
+				if (actions != null)
 					actions();
 			}
 		}
@@ -90,12 +93,14 @@ namespace ICSharpCode.SharpDevelop.Editor
 		public static RepoChangeWatcher AddWatch(string repositoryRoot, Action action)
 		{
 			RepoChangeWatcher watcher;
+			lock (watchers) {
 				if (!watchers.TryGetValue(repositoryRoot, out watcher)) {
 					watcher = new RepoChangeWatcher(repositoryRoot);
 					watchers.Add(repositoryRoot, watcher);
 				}
 				
 				watcher.actions += action;
+			}
 			return watcher;
 		}
 		
@@ -103,6 +108,7 @@ namespace ICSharpCode.SharpDevelop.Editor
 		
 		public void ReleaseWatch(Action action)
 		{
+			lock (watchers) {
 				actions -= action;
 				if (actions == null && !disposed) {
 					WorkbenchSingleton.MainWindow.Activated -= MainWindowActivated;
@@ -112,4 +118,5 @@ namespace ICSharpCode.SharpDevelop.Editor
 				}
 			}
 		}
+	}
 }