Never could figure it out. However, I uninstalled Pixinsight, deleted all folders, and removed all entries in the registry related to PixInsight. Then reinstalled PixInsight and Autointegrate. All works fine. Must have been a corrupted file someplace.
Thanks for reporting the problem, and happy to hear you do not see it again.
It looks like it happens when you use AutoIntegrate without a preview. I very rarely try that myself. The UIObject that it is complaining about is a valid UIObject that was created just a few lines earlier.
I have seen some weird problems when objects are left in a bad state in the PixInsight Javascript engine. Usually running a command run --reset in the Process Console fixes these problems. If it is something else then a full reset like you did might be the best choice.
Anyway I will check later today if I can reproduce the problem and if possible add an automatic test case for it.