Fixes VSTS Bug 958221: System.ArgumentNullException exception in

main/src/core/MonoDevelop.Core/MonoDevelop.Projects/ExtensionChain.cs

@@ -68,7 +68,7 @@ public void SetDefaultInsertionPosition (ChainedExtension insertBefore)
 
 		public IEnumerable<ChainedExtension> GetAllExtensions ()
 		{
-			return extensions;
+			return extensions ?? Enumerable.Empty<ChainedExtension> ();
 		}
 
 		public IDisposable BatchModify () => batchModifier = new BatchModifier (this);

main/src/core/MonoDevelop.Ide/MonoDevelop.Ide.Gui/Document.cs

@@ -130,23 +130,31 @@ void OnContentChanged ()
 
 		public T GetContent<T> (bool forActiveView) where T : class
 		{
+			if (IsDisposed)
+				return null;
 			return (T)GetContent (forActiveView, typeof (T));
 		}
 
 		public T GetContent<T> () where T : class
 		{
+			if (IsDisposed)
+				return null;
 			return GetContent<T> (false);
 		}
 
 		public IEnumerable<T> GetContents<T> (bool forActiveView) where T : class
 		{
+			if (IsDisposed)
+				return Enumerable.Empty<T> ();
 			if (forActiveView)
 				return view.GetActiveControllerHierarchy ().Select (controller => controller.GetContents<T> ()).SelectMany (c => c);
 			return GetControllersForContentCheck ().Select (controller => controller.GetContents<T> ()).SelectMany (c => c);
 		}
 
 		public IEnumerable<T> GetContents<T> () where T : class
 		{
+			if (IsDisposed)
+				return Enumerable.Empty<T> ();
 			return GetContents<T> (false);
 		}
 
@@ -637,8 +645,11 @@ void Window_CloseRequested (object sender, EventArgs e)
 			Close ().Ignore ();
 		}
 
+		public bool IsDisposed { get; private set; }
+
 		void Dispose ()
 		{
+			IsDisposed = true;
 			DocumentRegistry.Remove (this);
 			callbackRegistration?.Dispose ();
 			var workspace = Runtime.PeekService<RootWorkspace> ();