Fix forum-12634: ArgumentNullException on Right click on Project Upgrade tab and call "Navigate to file"

src/Main/Base/Project/Src/Commands/FileTabStripCommands.cs

@@ -81,8 +81,11 @@ namespace ICSharpCode.SharpDevelop.Commands.TabStrip
 		public override void Run()
 		{
 			var projectBrowser = (ProjectBrowserPad.Instance.Control as ProjectBrowserPanel).ProjectBrowserControl;
-			projectBrowser.SelectFileAndExpand(((IWorkbenchWindow)Owner).ActiveViewContent.PrimaryFileName);
+			var fileName = ((IWorkbenchWindow)Owner).ActiveViewContent.PrimaryFileName;
-			projectBrowser.Focus();
+			if (fileName != null) {
+				projectBrowser.SelectFileAndExpand(fileName);
+				projectBrowser.Focus();
+			}
 		}
 	}
 	

src/Main/Base/Project/Src/Project/Solution/Solution.cs

@@ -37,6 +37,9 @@ namespace ICSharpCode.SharpDevelop.Project
 		#region Enumerate projects/folders
 		public IProject FindProjectContainingFile(string fileName)
 		{
+			if (fileName == null)
+				throw new ArgumentNullException("fileName");
+			
 			IProject currentProject = ProjectService.CurrentProject;
 			if (currentProject != null && currentProject.IsFileInProject(fileName))
 				return currentProject;