Removed WMS searchable layer can lead to app crash

[jacobwod]

Describe the bug
The Search tool allows us to configure WMS layers as searchable (using the corresponding WFS endpoint).

There is a problem that can occur if such a WMS layer is removed from layers definition but not removed from Search tool prior to the change. The issue is that the layer will not be visible in Admin anymore (hence not removable), but its ID will still exist in the selectedSources array, which can lead to this:

Proposed solution
As Admin is not capable of removing the erroneous entry, we have to handle it differently. I'll take care of Client, so it won't crash. In addition, I'll print a log message in console that tells admins to take a look into their configs and (manually) remove references to the missing layer.