completeError method Null safety

Complete future with an error.

Calling complete or completeError must be done at most once.

Completing a future with an error indicates that an exception was thrown while trying to produce a value.

If error is a Future, the future itself is used as the error value. If you want to complete with the result of the future, you can use:

thisCompleter.complete(theFuture)

or if you only want to handle an error from the future:

theFuture.catchError(thisCompleter.completeError);