[BUG] Fix NullReferenceException at ShowSubMenu

[Hofknecht]

reported via mail in version 1.3.1.0:

System.NullReferenceException: Object reference not set to an instance of an object. at SystemTrayMenu.Business.Menus.ShowSubMenu(Menu menuToShow) in D:\a\1\s\Business\Menus.cs:line 1058 at SystemTrayMenu.Business.Menus.<.ctor>g__CreateAndShowLoadingMenu|22_15(RowData rowData) in D:\a\1\s\Business\Menus.cs:line 162 at SystemTrayMenu.Business.Menus.<.ctor>g__StartLoadMenu|22_2(RowData rowData) in D:\a\1\s\Business\Menus.cs:line 147 at SystemTrayMenu.Handler.WaitToLoadMenu.CallOpenMenuNow() in D:\a\1\s\Business\WaitToLoadMenu.cs:line 183 at SystemTrayMenu.Handler.WaitToLoadMenu.WaitStartLoad_Tick(Object sender, EventArgs e) in D:\a\1\s\Business\WaitToLoadMenu.cs:line 161 at System.Windows.Forms.Timer.TimerNativeWindow.WndProc(Message

reported via mail in version 1.3.1.0:

System.NullReferenceException: Object reference not set to an instance of an object. at SystemTrayMenu.Business.Menus.ShowSubMenu(Menu menuToShow) at SystemTrayMenu.Business.Menus.<.ctor>g__LoadSubMenuCompleted|22_14(Object senderCompleted, RunWorkerCompletedEventArgs e) at System.ComponentModel.BackgroundWorker.OnRunWorkerCompleted(RunWorkerCompletedEventArgs e) at System.ComponentModel.BackgroundWorker.AsyncOperationCompleted(Object arg)

Fixed in 1.3.1.0 together in commit with #443

see 47f5c32

see Menus.cs:line 1058