[Mac] Check for NSOpenSavePanelUrlEventArgs.NewDirectoryUrl being NSNull

If the file search inside an NSSavePanel is triggered on Catalina
for some reason NSOpenSavePanelUrlEventArgs.NewDirectoryUrl might
be NSNull and not NSUrl. Trying to access any of its properties
results in a runtime crash.

Fixes VSTS #1015827

Author: sevoku

main/src/addins/MacPlatform/Dialogs/MacOpenFileDialogHandler.cs

@@ -1,4 +1,4 @@
-// 
+// 
 // MacSelectFileDialogHandler.cs
 //  
 // Author:
@@ -76,29 +76,29 @@ public bool Run (OpenFileDialogData data)
 			try {
 				using (var panel = CreatePanel (data, out var state)) {
 					bool pathAlreadySet = false;
-					//TODO: DidChangeToDirectory is broken on Catalina
-					if (MacSystemInformation.OsVersion < MacSystemInformation.Catalina) {
-						var panelClosedToken = panelClosedSource.Token;
-						panel.DidChangeToDirectory += (sender, e) => {
-							var directoryPath = e.NewDirectoryUrl?.AbsoluteString;
-							if (string.IsNullOrEmpty (directoryPath))
-								return;
-							var selectedPath = data.OnDirectoryChanged (this, directoryPath);
-							if (selectedPath.IsNull)
-								return;
-							data.SelectedFiles = new FilePath [] { selectedPath };
-							pathAlreadySet = true;
+					var panelClosedToken = panelClosedSource.Token;
+					panel.DidChangeToDirectory += (sender, e) => {
+						// HACK: On Catalina e.NewDirectoryUrl might be NSNull instead of null
+						if (e.NewDirectoryUrl == null || ((NSObject)e.NewDirectoryUrl) is NSNull)
+							return;
+						var directoryPath = e.NewDirectoryUrl.AbsoluteString;
+						if (string.IsNullOrEmpty (directoryPath))
+							return;
+						var selectedPath = data.OnDirectoryChanged (this, directoryPath);
+						if (selectedPath.IsNull)
+							return;
+						data.SelectedFiles = new FilePath [] { selectedPath };
+						pathAlreadySet = true;
 
-							// We need to call Cancel on 1ms delay so it's executed after DidChangeToDirectory event handler is finished
-							// this is needed because it's possible that DidChangeToDirectory event is executed while dialog is opening
-							// in that case calling .Cancel() leaves dialog in weird state...
-							// Fun fact: DidChangeToDirectory event is called from Open on 10.12 but not on 10.13
-							System.Threading.Tasks.Task.Delay (1).ContinueWith (delegate {
-								if (!panelClosedToken.IsCancellationRequested)
-									panel.Cancel (panel);
-							}, panelClosedToken, System.Threading.Tasks.TaskContinuationOptions.None, Runtime.MainTaskScheduler);
-						};
-					}
+						// We need to call Cancel on 1ms delay so it's executed after DidChangeToDirectory event handler is finished
+						// this is needed because it's possible that DidChangeToDirectory event is executed while dialog is opening
+						// in that case calling .Cancel() leaves dialog in weird state...
+						// Fun fact: DidChangeToDirectory event is called from Open on 10.12 but not on 10.13
+						System.Threading.Tasks.Task.Delay (1).ContinueWith (delegate {
+							if (!panelClosedToken.IsCancellationRequested)
+								panel.Cancel (panel);
+						}, panelClosedToken, System.Threading.Tasks.TaskContinuationOptions.None, Runtime.MainTaskScheduler);
+					};
 
 					panel.SelectionDidChange += delegate {
 						var selection = MacSelectFileDialogHandler.GetSelectedFiles (panel);