search_home route raises AttributeError for missing search components

[jacobtylerwalls]

• Visit /search?search-view=nonexistent

We could respond with 404 instead of letting AttributeError escape.

Will send a fix that I cooked up while following along with the walk through from the dev meeting, which I very much enjoyed :-)

Traceback (most recent call last):
  File "/Users/jwalls/prj/arches/arches/app/views/search.py", line 84, in get
    search_components = searchview_instance.get_searchview_filters()
                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Exception Type: AttributeError at /search
Exception Value: 'NoneType' object has no attribute 'get_searchview_filters'