Fix crashing preview window

Fixes #1274 

* Allow the preview and config screens to work with world generators that require asset loading (like the Heightmap).  
* Fix the preview world screen to reset when opening with a different generator selected (it was never reinitializing with the new generator).  
* Does not crash the game when a world generator does not play nice with the preview window.
I think that the exception should be logged here. Silent failures are hard to track down.