Call to a member function cache_reset() on a non-object: i18n_string_locale_translate_edit_form_submit

Problem/Motivation

When submitting translated strings the submit handler fails due to call to a member function of a non-object.

Proposed resolution

Check if i18n_string_get_by_lid($lid) returns a valid object.
The same is done in i18n_string_locale_translate_edit_form() so I think this can be done here the same way.
But I have no overview of what you're doing here at all :)