Fix ArgumentNullException due to null filename when stepping in debugger.

src/AddIns/Debugger/Debugger.AddIn/Service/WindowsDebugger.cs

@@ -661,6 +661,8 @@ namespace ICSharpCode.SharpDevelop.Services
 		
 		public override void JumpToCurrentLine(string sourceFullFilename, int startLine, int startColumn, int endLine, int endColumn)
 		{
+			if (string.IsNullOrEmpty(sourceFullFilename))
+				return;
 			IViewContent viewContent = FileService.OpenFile(sourceFullFilename);
 			if (viewContent != null) {
 				IPositionable positionable = viewContent.GetService<IPositionable>();

src/Main/SharpDevelop/Workbench/DisplayBinding/DisplayBindingService.cs

@@ -99,6 +99,8 @@ namespace ICSharpCode.SharpDevelop.Workbench
 		public IDisplayBinding GetBindingPerFileName(FileName filename)
 		{
 			SD.MainThread.VerifyAccess();
+			if (filename == null)
+				return null;
 			if (FileUtility.IsUrl(filename)) {
 				// The normal display binding dispatching code can't handle URLs (e.g. because it uses Path.GetExtension),
 				// so we'll directly return the browser display binding.
@@ -115,6 +117,8 @@ namespace ICSharpCode.SharpDevelop.Workbench
 		{
 			SD.MainThread.VerifyAccess();
 			
+			if (filename == null)
+				return null;
 			string defaultCommandID = displayBindingServiceProperties.Get("Default" + Path.GetExtension(filename).ToLowerInvariant(), string.Empty);
 			if (!string.IsNullOrEmpty(defaultCommandID)) {
 				foreach (DisplayBindingDescriptor binding in bindings) {

src/Main/SharpDevelop/Workbench/FileService.cs

@@ -304,6 +304,8 @@ namespace ICSharpCode.SharpDevelop.Workbench
 		/// <inheritdoc/>
 		public IViewContent OpenFile(FileName fileName, bool switchToOpenedView)
 		{
+			if (fileName == null)
+				throw new ArgumentNullException("fileName");
 			LoggingService.Info("Open file " + fileName);
 			
 			IViewContent viewContent = GetOpenFile(fileName);