Fix cross-project milestone ref with invalid project

lib/banzai/filter/milestone_reference_filter.rb

@@ -17,9 +17,7 @@ module Banzai
         return super(text, pattern) if pattern != Milestone.reference_pattern
 
         text.gsub(pattern) do |match|
-          project = project_from_ref($~[:project])
-          params = milestone_params($~[:milestone_iid].to_i, $~[:milestone_name])
-          milestone = project.milestones.find_by(params)
+          milestone = find_milestone($~[:project], $~[:milestone_iid], $~[:milestone_name])
 
           if milestone
             yield match, milestone.iid, $~[:project], $~
@@ -29,6 +27,22 @@ module Banzai
         end
       end
 
+      def find_milestone(project_ref, milestone_id, milestone_name)
+        project = project_from_ref(project_ref)
+        return unless project
+
+        milestone_params = milestone_params(milestone_id, milestone_name)
+        project.milestones.find_by(milestone_params)
+      end
+
+      def milestone_params(iid, name)
+        if name
+          { name: name.tr('"', '') }
+        else
+          { iid: iid.to_i }
+        end
+      end
+
       def url_for_object(milestone, project)
         h = Gitlab::Routing.url_helpers
         h.namespace_project_milestone_url(project.namespace, project, milestone,
@@ -43,14 +57,6 @@ module Banzai
             html_safe
         end
       end
-
-      def milestone_params(iid, name)
-        if name
-          { name: name.tr('"', '') }
-        else
-          { iid: iid }
-        end
-      end
     end
   end
 end