app fails to load when map contains reference to layer that no longer exists

[allisonmuise]

App throws a type error and fails to load the list of polling features when the app is configured to display a map that contains one or more layers which no longer exist. This behavior is seen even when the deleted layer is NOT configured as the polling layer.

Repro steps:

1. Create a map that contains 2 hosted feature layers, one of which meets the requirements for a Polling layer.
2. Save the map.
3. Delete the AGOL item for the non-polling layer
4. From the Content window, create a new CS Polling app.
5. Configure the app to use the map created in step 1
6. Configure the app to use the layer that was NOT deleted.
7. Open the app and notice that the panel spins and fails to load.

Workaround: Use a valid map...