Return nil instead of crashing when copyCheckedString is passed a NULL C string.  Report errors for missing citekey and type.
Return nil instead of crashing when copyCheckedString is passed a NULL C string.  Report errors for missing citekey and type.

JackieXie168/skim,JackieXie168/skim,JackieXie168/skim,JackieXie168/skim,JackieXie168/skim