Even when the global _capture_stderr is set, the functions allow users to not capture stderr (e.g., by using devnull). In these cases, sys.stderr.write fails because stderr is None. An example is grass.script.db.db_connection where db.connect regularly fails during grass.script.setup.finish when there is no database connection. With this fix, the function no longer fails and correctly handles the error state with except CalledModuleError.

The issue with the original code is visible in the code itself: _capture_stderr is used in a condition to set stderr=PIPE, but that's conditional on values in kwargs. However, this second condition is not considered when output stderr value is used later on.