Add NullArgumentException for Run with main window

src/Avalonia.Controls/Application.cs

@@ -243,14 +243,14 @@ namespace Avalonia
             Dispatcher.UIThread.InvokeAsync(
                 () =>
                     {
-                        if (mainWindow == null)
+                        if (MainWindow != null)
                         {
                             return;
                         }
 
-                        if (MainWindow != null)
+                        if (mainWindow == null)
                         {
-                            return;
+                            throw new ArgumentNullException(nameof(mainWindow));
                         }
 
                         if (!mainWindow.IsVisible)
@@ -283,7 +283,7 @@ namespace Avalonia
             if (!IsExiting)
             {
                 OnExit?.Invoke(this, EventArgs.Empty);
-            }       
+            }
         }
 
         /// <summary>

tests/Avalonia.Controls.UnitTests/ApplicationTests.cs

@@ -1,6 +1,7 @@
 // Copyright (c) The Avalonia Project. All rights reserved.
 // Licensed under the MIT license. See licence.md file in the project root for full license information.
 
+using System;
 using System.Collections.Generic;
 using Avalonia.UnitTests;
 using Xunit;
@@ -103,5 +104,14 @@ namespace Avalonia.Controls.UnitTests
                 Assert.Empty(Application.Current.Windows);
             }
         }
+
+        [Fact]
+        public void Throws_ArgumentNullException_On_Run_If_MainWindow_Is_Null()
+        {
+            using (UnitTestApplication.Start(TestServices.StyledWindow))
+            {
+                Assert.Throws<ArgumentNullException>(() => { Application.Current.Run(null); });
+            }
+        }
     }
-}
+}