Collect crashes when the offline layer is changed to 'None'

[mmarciniak90]

Software and hardware versions

Collect master

Problem description

Collect crashes when the offline layer is changed to 'None'

Steps to reproduce the problem

Preconditions:

1. User uses Mapbox
2. The offline layer is set

Use case 2:

1. User fills the form with some Geo widget with map
2. User opens layers dialog
3. User selects None
4. Collect crashes

Expected behavior

The offline layer is disabled

Other information

Noticed during testing #3447
It's hard to define stacktrace.
Here is possible stacktrace but I'm not sure