Update mainWindow name

StabilityMatrix/App.xaml.cs

@@ -44,10 +44,10 @@ namespace StabilityMatrix
         /// <param name="args">Details about the launch request and process.</param>
         protected override void OnLaunched(Microsoft.UI.Xaml.LaunchActivatedEventArgs args)
         {
-            m_window = new MainWindow();
-            m_window.Activate();
+            mainWindow = new MainWindow();
+            mainWindow.Activate();
         }
 
-        private Window m_window;
+        private Window mainWindow;
     }
 }