In some callback functions there is a bug which causes an NPE if an error occurs during command execution. E.g. in PushImageResultCallback.java:43

    if (latestItem == null || latestItem.isErrorIndicated()) {
        throw new DockerClientException("Could not push image: " + latestItem.getError());
    }

If latestItem is null, the subsequent call to latestItem.getErrror() throws an NPE.

Similar bugs are in these classes:
BuildImageResultCallback.java
PullImageResultCallback.java
PushImageResultCallback.java