Extra field parsing was crashing when it stumbled across types!=string

I dont know why this went into the old pullrequest...
But as you can see in the files changed it is not much code.
let me know what you think 
johannes
This seems very special case - could the name be built into the field request so it pulls out the right info automatically?

For example, you would do:
    task_extra_display_fields: ["project.Project.name"]
instead of:
    task_extra_display_fields: ["name"]
when getting the project for a shot or other entity?

Then the fix for the bug could be reduced to:
   extra_data = [d.get(f) for f in extra_fields]
   name_extension = ", ".join([str(item) for item in extra_data if item != None])

Would that work?