Edited Issue: Designer: "Add a Condition" => "out of range" error [783]

Note: This does not apply to the Designer which was shipped in VS2012 -- only to the private codebase maintained by moozzyk.

Repro steps:
1. Add new EDMX from existing database.
2. Go to mapping details for entity mapped to an existing table.
3. Click "<Add a Condition>" and attempt to add a condition.

Error:
"Specified argument was out of the range of valid values. Parameter name: row"