Remember that the layer set used by a layout view is always saved with the plan that the view was originally sent to layout from. For this reason, you should avoid deleting layer sets from a plan unless you are absolutely sure that they are not being used by layout views.
You can find out which layer set a layout view is using by selecting it and clicking the Layout Box Layers edit button.
If you select a layout view and click the Open View edit button, you will return to the original view in the plan file. Bear in mind that the layout view's layer set will be active. If you make changes to its settings, the view on the layout page will be affected.