Fix extra argument value display

[duranb]

Resolves #1125

To test:

1. Upload a Banananation model
2. Create a plan using that model
3. Add a ParameterTest activity and explicitly set one of the struct/array values
4. Go to localhost:8080 and navigate to the activity_type table
5. Edit the row of the ParameterTest activity type that you have added to the plan
6. In the parameters column of that row, change the name of one of the struct/array parameters that you explicit had set in the plan and save
7. Navigate back to the plan and select the directive of the activity type that you modified
8. Verify that an Extraneous Parameter is displayed and that the value is not [object Object] in the input field