Background holder has dangling reference to removed viewport

[Pheubel]

The problem

When switching from a background with a custom scene to a background with an invalid scene or no custom scene, it does not reset the 'current_viewport' meta data. This causes problems later on as when it is fetched instead of null, it is a reference to a freed resource.

To Reproduce
Steps to reproduce the behavior:

1. create timeline
2. add background with custom scene
3. add background with no custom scene
4. add another background with a custom scene.
5. add text events in between to prevent the timeline from speeding through it.
6. test the timeline.

Expected behavior
There should be no halt caused by a freed resource, instead the timeline should have moved onto the next event.

Screenshots

System (please complete the following information):

• OS: [e.g. Windows, Linux]
• Godot Version: [e.g. 3.2.3]
• Dialogic Version: 2.11 (main - commit b96d4aa)

Solution

When no new viewport is available, it should remove the meta data for the current_scene, so that when it attempts to retrieve it will get null as a default value instead. That way the rest of the event will flow as expected.