v2 null reference in FindAndOpenChildrenMenuByHotkey when menu has separators

[tznind]

Describe the bug
When pressing an alt shortcut on a menu with a separator you get null reference exception.

This is because separators are represented as null in menu items.

To Reproduce

1. Launch UICatalog
2. Click on the Diagnostics menu
3. Press Alt+D (or any other letter)
4. Get hard crash

Screenshots

Null reference 

>	Terminal.Gui.dll!Terminal.Gui.MenuBar.FindAndOpenChildrenMenuByHotkey(Terminal.Gui.KeyEvent kb, Terminal.Gui.MenuItem[] children) Line 1847	C#
 	Terminal.Gui.dll!Terminal.Gui.MenuBar.FindAndOpenMenuByHotkey(Terminal.Gui.KeyEvent kb) Line 1828	C#
 	Terminal.Gui.dll!Terminal.Gui.MenuBar.ProcessHotKey(Terminal.Gui.KeyEvent kb) Line 1947	C#
 	Terminal.Gui.dll!Terminal.Gui.View.ProcessHotKey(Terminal.Gui.KeyEvent keyEvent) Line 361	C#
 	Terminal.Gui.dll!Terminal.Gui.Application.ProcessKeyEvent(Terminal.Gui.KeyEvent ke) Line 1264	C#
 	Terminal.Gui.dll!Terminal.Gui.WindowsDriver.ProcessInput(Terminal.Gui.WindowsConsole.InputRecord inputEvent) Line 984	C#
 	Terminal.Gui.dll!Terminal.Gui.WindowsMainLoop.Terminal.Gui.IMainLoopDriver.Iteration() Line 1932	C#
 	Terminal.Gui.dll!Terminal.Gui.MainLoop.RunIteration() Line 339	C#
 	Terminal.Gui.dll!Terminal.Gui.Application.RunMainLoopIteration(ref Terminal.Gui.RunState state, ref bool firstIteration) Line 633	C#
 	Terminal.Gui.dll!Terminal.Gui.Application.RunLoop(Terminal.Gui.RunState state) Line 615	C#
 	Terminal.Gui.dll!Terminal.Gui.Application.Run(Terminal.Gui.Toplevel view, System.Func<System.Exception, bool> errorHandler) Line 498	C#
 	Terminal.Gui.dll!Terminal.Gui.Application.Run<UICatalog.UICatalogApp.UICatalogTopLevel>(System.Func<System.Exception, bool> errorHandler, Terminal.Gui.ConsoleDriver driver, Terminal.Gui.IMainLoopDriver mainLoopDriver) Line 443	C#
 	UICatalog.dll!UICatalog.UICatalogApp.RunUICatalogTopLevel() Line 214	C#
 	UICatalog.dll!UICatalog.UICatalogApp.Main(string[] args) Line 115	C#

Desktop (please complete the following information):

• OS: [e.g. iOS]
• Browser [e.g. chrome, safari]
• Version [e.g. 22]

Smartphone (please complete the following information):

• Device: [e.g. iPhone6]
• OS: [e.g. iOS8.1]
• Browser [e.g. stock browser, safari]
• Version [e.g. 22]

Additional context
Add any other context about the problem here.