ImageFormater and Page Module throw fatal error after deleting unused image style

Problem Motivation

When deleting an unused image style, pages routed by the page module throw an php exception. (see pdf attached)

How to reproduce

1. install page module
2. create a custom route with the page module, place blocks there
3. create an custom image Style (my_new_image_style)
4. use this style in a content type (manage display, format image styles)
5. check if this style is used on the custom route
6. reassign an other image style to the image previously holding my_new_image_style
7. delete my_new_image_style
8. browse back to the custom route, find the fatal error

Discussion

Although all custom routes are affected the drupal backend and other single node content pages were rendered correctly. It seems like the imageFormater got called although the pages not even contained a single image.