A trio of snippets pane fixes

[zadjii-msft]

1. Don't crash on a cmdpal "duplicate pane" of a snippets pane
   • Found while trying to solve bug the third.
   • "Duplicate pane" with a snippets pane would crash. This was due to us attempting to PreviewText when there was no buffer yet. (_activeBuffer() strikes again)
2. dismiss the preview from cmdpal correctly too
   • Again while looking for part the third, I hit this
   • I have a sendInput(input: "a") command. This is the first command in the palette. And opening a new pane would... preview that command in the new pane? weird. Moving the line in CommandPalette::_close fixes this
3. Don't crash when we're restoring a snippets pane and there's a bunch of windows
   • This was the real bug I was trying to fix
   • Looks like if you have enough panes & windows, there's enough of a delay between ctoring a snippets pane and actually calling _UpdateSettings on it, that the XAML loads and tries to bind to _allTasks, which hadn't been constructed yet
   • closes Crash on launch with a snippets pane, and multiple windows(?) #17793