Editor breaks if trying to access a panel that has been removed using advanced editor

[RyanCoulsonCA]

Description

From @dnlnashed in #317:

When removing a panel using the advanced editor, if you click on the panel side that's been removed the slide editor ends up breaking

Steps to reproduce

1. Go to the demo page and load a product.
2. Click on a slide and go to the advanced editor tab.
3. Remove the second panel config and then try to click on the Right panel tab.
4. Notice that nothing happens and an error appears in the console.

Expected behavior

Two options: Either the tab should disappear immediately after the panel config is removed, or we should at least check if the panel still exists when the user clicks on the tab.

Actual behavior

Nothing happens and the editor breaks.