Better error message if key is not specified in ATTRIBUTE_TYPES (#1762)

[key] → fetch(key)

to provide better error description if something goes wrong.
Originally, we get "NoMethodError: undefined method
`associative?' for nil:NilClass"

lib/administrate/base_dashboard.rb

@@ -91,7 +91,7 @@ def attribute_not_found_message(attr)
 
     def attribute_includes(attributes)
       attributes.map do |key|
-        field = self.class::ATTRIBUTE_TYPES[key]
+        field = attribute_type_for(key)
 
         key if field.associative?
       end.compact