Calculation leads to runtime error when switching to structurally different language

[chris-steele]

If the user switches to a structurally different language it is possible for a runtime error to occur. The error will occur in the following scenario:

1. the user opens a page in a particular language
2. whilst on the page the user switches to a language that does not have the page

The cause appears to be down to the plugin using the current location identifier to locate the page model. As the page model does not exist a reference error occurs.

[oliverfoster]

I believe this is an almost identical bug. adaptlearning/adapt-contrib-core#568