Dispose windows before exiting

QuickLook/App.xaml.cs

@@ -127,9 +127,12 @@ namespace QuickLook
         {
             if (_isFirstInstance)
             {
+                _isRunning.ReleaseMutex();
+
                 PipeServerManager.GetInstance().Dispose();
                 TrayIconManager.GetInstance().Dispose();
                 BackgroundListener.GetInstance().Dispose();
+                ViewWindowManager.GetInstance().Dispose();
             }
         }
 

QuickLook/ViewWindowManager.cs

@@ -48,6 +48,7 @@ namespace QuickLook
         public void Dispose()
         {
             StopFocusMonitor();
+            ClosePreview();
         }
 
         internal void InvokeRoutine(KeyEventArgs kea, bool isKeyDown)