Fix #19

Author: Hexer10

UI/MainWindow.xaml.cs

@@ -201,7 +201,7 @@ private void DockingManager_ActiveContentChanged(object sender, EventArgs e)
 
         private void DockingManager_DocumentClosing(object sender, Xceed.Wpf.AvalonDock.DocumentClosingEventArgs e)
         {
-            ((EditorElement) e.Document.Content).Close();
+            (e.Document.Content as EditorElement)?.Close();
             UpdateWindowTitle();
         }