Storytelling collapse when the layer is deleted

[KaWaite]

Describe the bug
When the layer that is registered as the storytelling story is deleted, it remains as the story though it doesn't exist on the digital Earth.

To Reproduce
Steps to reproduce the behavior:

1. add a couple of layers to the earth
2. activate storytelling
3. add them to the storytelling list
4. delete a layer from the outline window
5. play with storytelling
6. storytelling will collapse since there isn't the layer which storytelling expects to have

11/10/2021 KaWaite

• Seems storytelling's property references the Layers (stories), which on Layer deletion of course doesn't search through all widgets that might reference that Layer and delete references to it. Either do a sweep (probably not a good solution) or create a warning before Layer deletion.