Creating a Sales Order Using a Custom Role Throws "JS_EXCEPTION TypeError 'length' is null or not an object"

1. Navigate to Setup>Users/Roles>Manage Roles>Edit Custom Role>Permissions tab>Custom Record subtab and set the following custom records to at least "View" level:

Contract Renewals Preferences
Contract
Contract Items

2. If Step 1 does not work, make the Standard Contract Renewals form the preferred form for the role under Setup > Users/Roles > Manage Roles > Edit Custom Role > Forms tab > Transactions subtab > Mark Preferred Checkbox for "Sales Order - Contract Renewals".