body_might_complete_normally_catch_error

This 'onError' handler must return a value assignable to '{0}', but ends without returning a value.

The analyzer produces this diagnostic when the closure passed to the onError parameter of the Future.catchError method is required to return a non-null value (because of the Futures type argument) but can implicitly return null.

The following code produces this diagnostic because the closure passed to the catchError method is required to return an int but doesn't end with an explicit return, causing it to implicitly return null:

void g(Future<int> f) {
  f.catchError((e, st) {});
}

If the closure should sometimes return a non-null value, then add an explicit return to the closure:

void g(Future<int> f) {
  f.catchError((e, st) {
    return -1;
  });
}

If the closure should always return null, then change the type argument of the Future to be either void or Null:

void g(Future<void> f) {
  f.catchError((e, st) {});
}