Fix a NullReferenceException that was discovered while fixing EDT-785.
EDT-785: Fix a NullReferenceException that was caused while working on EDT-765. During refactoring, code was removed that had been ensuring that all terms were present in all languages while opening an ADL archetype. Any missing term in a particular language was given "#Error#" as its text and description in that language. By removing this code, an archetype could not be opened if terms were missing in any languages.

This is fixed by reinstating the previous behaviour, with following changes:
* If the term exists in the primary language, then the text, description and comment are set to the same values as in the primary language in the normal manner (e.g., "* Something (en)".
* If the term doesn't exist in the primary language, the description is left empty and the text is initialised to "#Missing#" rather than "#Error#".