Intervention Creating Error when there is 1 recommended monitor (and it is missing probe definition)

creating an intervention with 1 recommended monitor that has no probe definition (either nil, or nonexisting)

Confirm:
1. RecommendedMonitors are delete_all for intervention_definition and probe_definition
2. No nil recommended monitors are created when I create a district with dpi or mmsd content)
3. Even if this somehow happens, it doesn't fail.
4. The method in question has unit tests.

(from [d421ab1b3a33a2c3455a1617c763ab41e05e4c08]) [#519] make sure recommended monitors with nil or missing progress monitors do not break when autoassigning probe http://github.com/vegantech/sims/commit/d421ab1b3a33a2c3455a1617c76...

(from [be0a2f381219692a4ae20bd92856c101d52a8ca5]) [#519] no more nil recommended_monitors are created when district content is generated http://github.com/vegantech/sims/commit/be0a2f381219692a4ae20bd9285...