Fix crash when onboarding is shown before window is loaded (mac)

Setting positioningRect when window is not yet visible may crash the app with
NSInternalInconsistencyException "window must exist".
Moved this line to delegate method, so it'll be called only after popover show
method executes successfully.

src/ui/osx/TogglDesktop/Features/Onboarding/OnboardingViewController.swift

@@ -87,7 +87,6 @@ final class OnboardingViewController: NSViewController {
         preparePopoverContentView(with: payload)
 
         popover.show(relativeTo: presentView.bounds, of: presentView, preferredEdge: payload.preferEdges)
-        popover.positioningRect = payload.positioningRect(payload.hint, popover.positioningRect)
     }
 
     /// Dismiss all onboard view
@@ -161,6 +160,11 @@ extension OnboardingViewController: NSPopoverDelegate {
     func popoverWillClose(_ notification: Notification) {
         delegate?.onboardingViewControllerDidClose()
     }
+
+    func popoverWillShow(_ notification: Notification) {
+        guard let payload = payload else { return }
+        popover.positioningRect = payload.positioningRect(payload.hint, popover.positioningRect)
+    }
 }
 
 // MARK: OnboardingBackgroundViewDelegate