[#190] Work around a nil error introduced in c1cced5139

lib/redmine/helpers/gantt.rb

@@ -111,7 +111,11 @@ module Redmine
           end
         end
 
+        if @max_rows.present?
           rows > @max_rows ? @max_rows : rows
+        else
+          rows
+        end
       end
 
       # Returns the number of rows that will be used to list a project on