Crash on Startup - Null Timer

There is a bug introduced on this change I believe:

https://github.com/mono/MonoGame/commit/9a6d75d6e5aae8fbef92aaddd6f53c8a954571e7#diff-c9a47e02910a8951188f9eb0ac3615e8

For platforms that don’t return True from Platform.BeforeRun() the game timer will never be created. In the tick when it tries to access the elapsed time, it will be doing so on a null timer.