Fix scenarios init bug when loading project.
Fix scenarios init bug when loading project.

* The scenario collection doesn't yet exist on initialization of
the project model. We now wait until the scenario attributes
changes from null to the scenario collection, then set the first
scenario as the active scenario.

mmcfarland/model-my-watershed,lliss/model-my-watershed,WikiWatershed/model-my-watershed,kdeloach/model-my-watershed,mmcfarland/model-my-watershed,kdeloach/model-my-watershed,WikiWatershed/model-my-watershed,lliss/model-my-watershed,WikiWatershed/model-my-watershed,kdeloach/model-my-watershed,lliss/model-my-watershed,project-icp/bee-pollinator-app,lliss/model-my-watershed,mmcfarland/model-my-watershed,kdeloach/model-my-watershed,project-icp/bee-pollinator-app,lliss/model-my-watershed,kdeloach/model-my-watershed,project-icp/bee-pollinator-app,WikiWatershed/model-my-watershed,WikiWatershed/model-my-watershed,project-icp/bee-pollinator-app,mmcfarland/model-my-watershed,mmcfarland/model-my-watershed