Problem
Trying to use ExtJS with RubyOnRails on Heroku, gives the following error and a blank page in Firebug.

Ext.ux.grid has no properties
...
Line 141

Solution
In Heroku, you will probably have to manually upload the files from the ExtJS library, as well as the ext_scaffold plugin.
When you do that, two files that ext_scaffold needs, and which they should be located in ext_scaffold/assets/javascripts should be copied to public/javascripts.
The files are:

ext_datetime.js
ext_searchfield.js