Update TypeFieldElement.cs

If you add a custom type field definition in merchello.config you get an exception because the configuration property is missing.
Merge pull request #852 from calaa/patch-1

#M-383 Fixed
Merge pull request #851 from cssquirrel/1.4.0

Back Office Sales UI revisions
Merge remote-tracking branch 'merchello/1.4.0' into 1.4.0