fix: Use get method to handle the unknown urls

Usage of the square bracket notation raises KeyError for the unknown url.
Use get method providing a default value when the url is not found.

ckanext/dcatapchharvest/profiles.py

@@ -467,15 +467,15 @@ def _get_groups(self, subject):
                 if dcat_theme_url.startswith(OGD_THEMES_URI):
                     new_theme_url = dcat_theme_url.replace(
                         OGD_THEMES_URI, CHTHEMES_URI)
-                    eu_theme_url = unicode(
-                        eu_theme_mapping[URIRef(new_theme_url)][0])
+                    eu_theme_url = eu_theme_mapping.get(
+                        URIRef(new_theme_url), [None])[0]
 
                 # Case 2: We get a dcat-ap.ch theme (the same as the
                 #         opendata.swiss themes, but different base url). Get
                 #         the correct EU theme from the theme mapping.
                 elif dcat_theme_url.startswith(CHTHEMES_URI):
-                    eu_theme_url = unicode(
-                        eu_theme_mapping[URIRef(dcat_theme_url)][0])
+                    eu_theme_url = eu_theme_mapping.get(
+                        URIRef(new_theme_url), [None])[0]
 
                 # Case 3: We get an EU theme and don't need to look it up in
                 #         the mapping.