Blank sections and TypeError when modifying Sales Order

[Chris Horton]

I am trying to modify part of the Sales Order DocType in something based on ERPNext 3.6.1. Specifically, I'm trying to change a section for selecting a Project to link to to a table of Opportunity items. Any modification I make to the Sales Order DocType though seems to result in the New Sales Order page containing empty categories. I'm using Chrome, and in the JavaScript console I am also getting this error:

Uncaught TypeError: Cannot set property 'get_query' of undefined

You can see a screenshot of the issue and more of the stacktrace here: http://imgur.com/0xXHVv9

Any ideas on what's causing this and how I can resolve it? Thanks.