errorCallback method Null safety
Intercepts errors when added programmatically to a
When calling Completer.completeError, StreamController.addError,
or some Future constructors, the current zone is allowed to intercept
and replace the error.
Future constructors invoke this function when the error is received
directly, for example with Future.error, or when the error is caught
synchronously, for example with Future.sync.
There is no guarantee that an error is only sent through errorCallback
once. Libraries that use intermediate controllers or completers might
end up invoking errorCallback multiple times.
null if no replacement is desired. Otherwise returns an instance
of AsyncError holding the new pair of error and stack trace.
Although not recommended, the returned instance may have its
(AsyncError.error) be equal to
null in which case the error should be
replaced by a NullThrownError.
Custom zones may intercept this operation.
Implementations of a new asynchronous primitive that converts synchronous
errors to asynchronous errors rarely need to invoke errorCallback, since
errors are usually reported through future completers or stream
AsyncError? errorCallback(Object error, StackTrace? stackTrace);