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INavigation service becomes invalid
We use PageViewModel as a base for almost all our VMs. I initially saw this happen on Windows 8.1 app, where sometimes when we would go to navigate, the service IsValid would be false, Frame would be null and it would not go anywhere. We solved this by
adding our own property, that just gets INavigationService from the container.
I again have seen this happen on WP8, not sure what exactly gets it to that state, sometimes its after using the back button to go back, but I have also seen it where on first launch of the app on our first page it starts out not being valid.
Is there something I'm not handling right that is breaking this in your PageViewModel... seems odd that adding my own from container would be any different than that one.