How to handle when a term doesn't exists?

[ppazos]

When a term doesn't exists I'm getting an exception "Fatal error: Uncaught Error: Undefined class constant".

It happens when two lang files exist but one has missing translations.

[Philipp15b]

Yes, if the translation string is not available in a language file then the class does not contain the corresponding string. This is intended behaviour.

You could check if a term is defined in the current language using something like PHP's defined.
Hope that helps.